I've noticed this for a while, and It's quite a bizarre issue.

Often I've have Irish skills start to 'decay' down to 4/5 or 3/5, and when I go into the strengthen lesson for that particular unit, it'll immediately quit and mark it as gold. Sometimes, it'll only one or two vocabulary items to test, which isn't really enough to justify marking the lesson as decayed.

This bug is irritating as it's meaning that I'm accumulating XP and lingots that I really shouldn't be getting, and it also means I end up going into lessons and doing very short quizzes where I'd be better off using 'strengthen skills' and doing new material instead.

This may be because my Irish was strong enough to jump ahead a bit to start with, but even then, this really shouldn't be happening in the first place.