Have two Dolphin windows open. Drag from window 1 a file to window 2 (and do anything from the options). Now directly drag from window 2 a different file to window 1. The drag will fail, i.e. there is no option to move and selecting to copy will try to copy the previously dragged file of window 1. Some early debugging shows that the data is never requested to be sent by window 2 through the appropriate KWayland interfaces.
Related to/duplicate of bug https://bugs.kde.org/show_bug.cgi?id=383806
*** This bug has been marked as a duplicate of bug 383806 ***