In this position: WKc8, WBa3, WPg5; BKe8, BBf6, BPf5; White can mate in 1 by 1.g6#! But Popeye gives no solution.
If we add a BBh6 then Popeye gives 1.g6# as a solution.

Another test position: WKc8, WPc6; BKa8, BRb6, BSb7, BPa7; #1 by 1.c7#! But here too Popeye says no solution.
If we add BSd7 then 1.c7#! is accepted as a solution.

Is this a bug or am I missing something?

EDIT: It looks like Popeye is interpreting CtC as "...when two pieces of opposite colours stand adjacent to each other on the same rank, they transfer their powers to each other, while also retaining their original powers". But this seems to be against the rule in the related conditions: BackToBack, FaceToFace and Point Reflection, where the pieces lose their original powers.