Once an enemy pawn becomes queen, if you click on back move button, the queen-pawn will not be reverted to pawn. Reproducible: Didn't try
This appears to be a repeat of bug 314834. A patch for the bug is attached to that report. A second bug on pawn promotion is given in bug 332194, also with a patch attached.
*** This bug has been marked as a duplicate of bug 314834 ***