I got a trigger thumb over a year ago - I’d been on Letrozole for about nine years with no aches or joint pain at all. It wasn’t particularly painful but a nuisance. Medical advice was that I might need a splint. But after a few months it simply vanished - the trigger problem, not the thumb! So no idea what caused it, no idea what ended it. Nor any idea whether Letrozole caused it, although I understand it can. I would have thought however it would happen sooner, if Letrozole was the trigger (no pun intended).
No recurrence so far, cross fingers.