There is a new very nasty bug (well, it’s been there for a few weeks I suppose): when you flag a word as difficult during a review session, it rarely gets added to the list of difficult words.
If you finish a review session (which sadly is the only way of getting the list of reviewed words!) you’ll see the flash icon next to the words flagged as difficult during the session, but some may not really have been added to the list, i. e. the flag was not persisted.
The only way to make sure all words are added to the DW list is to unflag and flag them again and then check/compare the count of DW.
This is truly catastrophic for learning purposes!