Akka.Streams: Akka.Streams.Util.IteratorAdapter`1.Next() Collection modified on enumeration exception

[Aaronontheweb]

Version Information
Version of Akka.NET? 1.5.38
Which Akka.NET Modules? Akka.Streams

Describe the bug

Not entirely sure what causes it just yet, but here's the stack trace:

   at Akka.Streams.Util.IteratorAdapter`1.Next()
   at Akka.Streams.Implementation.Fusing.StatefulSelectMany`2.Logic.PushPull()
   at Akka.Streams.Implementation.Fusing.StatefulSelectMany`2.Logic.OnPush()
--- End of stack trace from previous location where exception was thrown ---
   at UserQuery.Main(), line 51

  at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Linq.Lookup`2.Create(IEnumerable`1 source, Func`2 keySelector, IEqualityComparer`1 comparer)
   at System.Linq.GroupedEnumerable`2.GetEnumerator()
   at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.MoveNext()
   at Akka.Streams.Util.IteratorAdapter`1.HasNext()

To Reproduce

If I take the same sample from #7512 and change it slightly:

.GroupBy(50, r => r.EntityId) // triggers exception

That will trigger the problem. You can modify the LINQPad sample I created for #7512 here: https://share.linqpad.net/bkgp72uf.linq

Expected behavior

Should process without error.

Actual behavior

It's slightly racy, but very frequently we get an "collection modified while enumerating" exception.