Messages sent to the wrong user sporadically when using notifyCommitters

[hudson-pierce]

Jenkins and plugins versions report

Environment

Jenkins: 2.355
OS: Linux - 4.15.0-176-generic
---
ace-editor:1.1
amazon-ecr:1.73.v741d474abe74
android-emulator:3.1.3
ant:475.vf34069fef73c
antisamy-markup-formatter:2.7
apache-httpcomponents-client-4-api:4.5.13-1.0
atlassian-jira-software-cloud:2.0.2
authentication-tokens:1.4
aws-credentials:191.vcb_f183ce58b_9
aws-java-sdk:1.12.215-339.vdc07efc5320c
aws-java-sdk-cloudformation:1.12.215-339.vdc07efc5320c
aws-java-sdk-codebuild:1.12.215-339.vdc07efc5320c
aws-java-sdk-ec2:1.12.215-339.vdc07efc5320c
aws-java-sdk-ecr:1.12.215-339.vdc07efc5320c
aws-java-sdk-ecs:1.12.215-339.vdc07efc5320c
aws-java-sdk-elasticbeanstalk:1.12.215-339.vdc07efc5320c
aws-java-sdk-iam:1.12.215-339.vdc07efc5320c
aws-java-sdk-logs:1.12.215-339.vdc07efc5320c
aws-java-sdk-minimal:1.12.215-339.vdc07efc5320c
aws-java-sdk-ssm:1.12.215-339.vdc07efc5320c
blueocean:1.25.5
blueocean-autofavorite:1.2.5
blueocean-bitbucket-pipeline:1.25.5
blueocean-commons:1.25.5
blueocean-config:1.25.5
blueocean-core-js:1.25.5
blueocean-dashboard:1.25.5
blueocean-display-url:2.4.1
blueocean-events:1.25.5
blueocean-git-pipeline:1.25.5
blueocean-github-pipeline:1.25.5
blueocean-i18n:1.25.5
blueocean-jira:1.25.5
blueocean-jwt:1.25.5
blueocean-personalization:1.25.5
blueocean-pipeline-api-impl:1.25.5
blueocean-pipeline-editor:1.25.5
blueocean-pipeline-scm-api:1.25.5
blueocean-rest:1.25.5
blueocean-rest-impl:1.25.5
blueocean-web:1.25.5
bootstrap4-api:4.6.0-5
bootstrap5-api:5.1.3-7
bouncycastle-api:2.26
branch-api:2.1046.v0ca_37783ecc5
build-discarder:60.v1747b0eb632a
build-timeout:1.21
build-token-root:1.9
buildresult-trigger:0.17
built-on-column:1.1
caffeine-api:2.9.3-65.v6a_47d0f4d1fe
checks-api:1.7.4
cloudbees-bitbucket-branch-source:773.v4b_9b_005b_562b_
cloudbees-folder:6.729.v2b_9d1a_74d673
codecommit-url-helper:0.0.3
command-launcher:84.v4a_97f2027398
conditional-buildstep:1.4.2
config-file-provider:3.10.0
convert-to-pipeline:1.0
copy-data-to-workspace-plugin:1.0
credentials:1129.vef26f5df883c
credentials-binding:523.vd859a_4b_122e6
declarative-pipeline-migration-assistant-api:1.5.2
display-url-api:2.3.6
docker-build-step:2.8
docker-commons:1.19
docker-custom-build-environment:1.7.3
docker-workflow:1.28
durable-task:496.va67c6f9eefa7
ec2:1.68
echarts-api:5.3.3-1
email-ext:2.89
envinject:2.866.v5c0403e3d4df
envinject-api:1.199.v3ce31253ed13
extensible-choice-parameter:1.8.0
external-monitor-job:191.v363d0d1efdf8
external-workspace-manager:1.2.3
favorite:2.4.1
font-awesome-api:6.1.1-1
generic-webhook-trigger:1.84
ghprb:1.42.2
git:4.11.3
git-client:3.11.0
git-parameter:0.9.17
git-server:1.11
github:1.34.3
github-api:1.303-400.v35c2d8258028
github-branch-source:1637.vd833b_7ca_7654
github-checks:1.0.18
github-oauth:0.39
github-pr-comment-build:67.va90f5cae5912
github-pullrequest:0.4.0
gradle:1.39.1
handlebars:3.0.8
handy-uri-templates-2-api:2.1.8-22.v77d5b_75e6953
htmlpublisher:1.30
icon-shim:3.0.0
jackson2-api:2.13.3-285.vc03c0256d517
javadoc:217.v905b_86277a_2a_
javax-activation-api:1.2.0-3
javax-mail-api:1.6.2-6
jaxb:2.3.6-1
jdk-tool:1.5
jenkins-design-language:1.25.5
jenkins-multijob-plugin:611.v9d3180d752e6
jira:3.7.1
jjwt-api:0.11.5-77.v646c772fddb_0
job-restrictions:0.8
jobConfigHistory:1148.v8607da_ef251e
jquery:1.12.4-1
jquery-detached:1.2.1
jquery3-api:3.6.0-4
jsch:0.1.55.2
junit:1119.va_a_5e9068da_d7
ldap:2.10
lockable-resources:2.15
log-parser:2.2
mailer:414.vcc4c33714601
matrix-auth:3.1.2
matrix-project:771.v574584b_39e60
maven-plugin:3.19
mdtool:0.1.1
mercurial:2.16.2
metrics:4.1.6.2
mina-sshd-api-common:2.8.0-18.vd98674ecd652
mina-sshd-api-core:2.8.0-18.vd98674ecd652
momentjs:1.1.1
monitoring:1.91.0
msbuild:1.30
multiple-scms:0.8
naginator:1.18.1
nested-view:1.25
next-build-number:1.8
node-iterator-api:1.5.1
nodejs:1.5.1
nodelabelparameter:1.11.0
nunit:0.27
okhttp-api:4.9.3-105.vb96869f8ac3a
pam-auth:1.8
parameterized-trigger:2.44
pipeline-aws:1.43
pipeline-build-step:2.18
pipeline-github:2.8-138.d766e30bb08b
pipeline-github-lib:38.v445716ea_edda_
pipeline-graph-analysis:195.v5812d95a_a_2f9
pipeline-groovy-lib:593.va_a_fc25d520e9
pipeline-input-step:448.v37cea_9a_10a_70
pipeline-milestone-step:101.vd572fef9d926
pipeline-model-api:2.2097.v33db_b_de764b_e
pipeline-model-definition:2.2097.v33db_b_de764b_e
pipeline-model-extensions:2.2097.v33db_b_de764b_e
pipeline-rest-api:2.24
pipeline-stage-step:293.v200037eefcd5
pipeline-stage-tags-metadata:2.2097.v33db_b_de764b_e
pipeline-stage-view:2.24
pipeline-utility-steps:2.12.2
plain-credentials:1.8
plugin-util-api:2.17.0
popper-api:1.16.1-3
popper2-api:2.11.5-2
port-allocator:1.8
postbuild-task:1.9
preSCMbuildstep:0.3
prometheus:2.0.11
pubsub-light:1.16
python:1.3
rebuild:1.34
resource-disposer:0.19
role-strategy:488.v0634ce149b_8c
run-condition:1.5
scm-api:608.vfa_f971c5a_a_e9
script-security:1175.v4b_d517d6db_f0
shared-workspace:1.0.2
slack:608.v19e3b_44b_b_9ff
snakeyaml-api:1.30.1
sse-gateway:1.25
ssh:2.6.1
ssh-agent:295.v9ca_a_1c7cc3a_a_
ssh-credentials:277.v95c2fec1c047
ssh-slaves:1.821.vd834f8a_c390e
sshd:3.242.va_db_9da_b_26a_c3
structs:318.va_f3ccb_729b_71
test-results-analyzer:0.3.5
text-finder:1.19
timestamper:1.18
token-macro:293.v283932a_0a_b_49
trilead-api:1.57.v6e90e07157e1
variant:1.4
windows-slaves:1.8.1
workflow-aggregator:581.v0c46fa_697ffd
workflow-api:1164.v760c223ddb_32
workflow-basic-steps:948.v2c72a_091b_b_68
workflow-cps:2725.v7b_c717eb_12ce
workflow-cps-global-lib:588.v576c103a_ff86
workflow-durable-task-step:1146.v1a_d2e603f929
workflow-job:1186.v8def1a_5f3944
workflow-multibranch:716.vc692a_e52371b_
workflow-scm-step:400.v6b_89a_1317c9a_
workflow-step-api:625.vd896b_f445a_f8
workflow-support:820.vd1a_6cc65ef33
ws-cleanup:0.42
xcode-plugin:2.0.17-565.v1c48051d46ef

What Operating System are you using (both controller, and any agents involved in the problem)?

Ubuntu

Reproduction steps

1. Add user to Jenkins with a matching Slack User ID configured in the user settings inside of Jenkins
2. Setup a Multibranch pipeline that builds based on changes to PRs
3. Using a slack send notification in the post stage of a Jenkins pipeline, like so:

    post {
        always {
            slackSend (
                channel: "#jenkinsbuilds",
                notifyCommitters: "true",
                color: 'good',
                message: "Message from Jenkins Pipeline"
            )
        }
    }

Expected Results

A slack notification should be sent to the user who made the commit.

Actual Results

On random occasions it will send a notification to a user who did not make any changes to the respective PR. Sometimes the user it sends the notification to has made no changes to the respective PR. This does not happen all the time, but rather sporadically without a clear pattern.

Anything else?

We have already double checked to ensure that the Jenkins users are configured properly with the correct Slack User IDs.