"Unfixable conflicts" when any, even empty, mappings file is applied

[ItsThosea]

I keep getting a full log of every mapping in Minecraft, followed by "unfixable conflicts", even if I supply an empty mapping file.
log (cut off by Intellij)
build.gradle
mappings.tiny
empty mappings.tiny that still causes error

gradle clean / gradle cleanloom doesn't work and also shows the error.
deleting .gradle and my loom cache does nothing
A temporary solution was to swap between loom 1.6 and 1.7 every time I made a gradle change, but this doesn't seem to work anymore. I've also tried pasting in a template build.gradle, refreshing, then repasting in my old one, which also doesn't seem to work anymore.

[ItsThosea]

This seems to only happen when using mojang mappings. Applying the same file on yarn works fine.

[LambdAurora]

I can confirm this issue on my part, with the simplest setup if attempting to override Mojang mappings using the layered mapping system will result in an Unfixable conflicts error with loom 1.7.

This error does not happen in loom 1.6, which leads me to think this is a regression.

This can be easily reproduced using the example mod and replacing the mappings with the following:

	mappings(loom.layered {
		officialMojangMappings { nameSyntheticMembers = false }
		mappings("net.fabricmc:yarn:1.21+build.9:v2")
	})