You looked at the queries, which are identical but the values are not. If you look at those, you can see that it's doing a "'Anonymous' LIKE 'en'"...
The reason is one of the dark places of the database layer, ensuring unique placeholders, which is done by recursively precompiling queries and passing around a shared object/state that returns a unique ID every time it's called for each query.
This works all fine until we get to the very special and dark case of UserSelection::entityQueryAlter() which manually compiles a query condition and passes the global default connection and the current query in for the placeholder ID stuff. Combine this with the count query, which is really a nested selected and we're now inside the inner query, which is *not* the one that was used for the compilation, the outer query would be, so we re-start the placer ID counter and get a _0 placeholder. That is the same as the language condition is using, so we re-use that placeholder and use the first value instead of the one that we're adding.
A possible workaround might be to preg_replace() the placeholder with one that is unique, but that's ugly (not that the whole piece of code there isn't already ugly). I suggest you get Crell and DamZ in here, because AFAIK they came up with all this preExecute() fun (I was involved too, but not as much as they).
As per @Crell (in #2144377-135: Entity reference autocomplete lists entity labels only in current content language → ), pre-compiling a query is not supported and need to be fixed in UserSelection::entityQueryAlter
Prosed solution hints from @Crell on IRC:
01:22 Crell: That plugin needs to get the DB connection injected properly.
01:22 Crell: And then not use db_and()/db_or().
01:22 Crell: And then not precompile the string and hope it works.
01:23 Crell: Actually… Hm, can you get the connection from the $query?
01:23 Crell: I could see an argument that you may need to.
01:24 Crell: And this is that argument. Because there is, or at least there was an issue to add, methods to the connection for and/or so that the functions were not necessary.
Needs work
10.1 ✨
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This was tagged for issue summary update and tests from what I can still need to happen
Did not test yet.