Anecdata, datapoint of 1 personal experience: I had been using Opus 4.6 with no problems until yesterday, then it seemed like it was missing things constantly, introducing random bugs, forgetting basic stuff that was in its memory and skills and should never miss
Same thing happened on 2 separate sessions I had with Opus 4.6
Ended up switching to 5.5 and Fable, things improved instantly
This left me with a weird taste