Solved
I would confirm that you found a BUG in Prime 10.
My first guess was that it may be due to wrong labelling of the variables/arguments, but this seems not to be the case here.
The work around using the symbolic evaluation should not be necessary.
The bug seems to concern only functions with more than two arguments, using a function with just two arguments works as it should.

Here is another workaround not needing the symbolics. A local assignment does the trick!

Nonetheless its a BUG which should be fixed!
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