MergeUnion on types with different collators produces wrong result.
Problem can be reproduced with the following test (in EnumerableStringComparisonTest):
which fails with:
The problem is that, in case of different collators, the pre-requisite of the the MergeUnion (inputs sorted) is not fulfilled, since inputs are technically sorted, but not using the same sorting collator, so they are not comparable by the MergeUnion algorithm.
A possible solution could be not applying EnumerableMergeUnionRule in this case.
A more clever solution could be achieved if the rule pushes a Sort + Cast + input (and not just Sort + input) in case the input's key type differs collation-wise with the union's result type.