In general I find the difficult words feature to be useful, but a few words seem to stick around for way too long. The above example is frustrating - it only got in there because of a typo, and despite my having reviewed it correctly 20 times since it’s still there while other words come and go normally. Eventually they seem to disappear. I’ve also found manually unchecking them at the end of the review to only sometimes remove words - I think the two are related but that just may be because I’m more likely to uncheck sure words.
This is in the course “First 5000 words of Spanish” currently maintained by Xoviat; I’m not sure if it affects other courses.