They rolled out a new model a week ago which has a "bug" where in long chats it forgets to emit the tokens required for the UI to detect that it's reasoning. You can remind it that it needs to emit these tokens, which helps, or accept that it will sometimes fail to do it. I don't notice a deterioration in performance because it is still reasoning (you can tell by the nature of the output), it's just that those tokens aren't in <think> tags or whatever's required by the UI to display it as such.