Update and Reset: The Model After a Stroke

Hello!
Since yesterday, I’ve been dealing with a serious problem…
I use 4o (Plus user) for data analysis and generating textual reports. Until now, everything has been great – the insights were deep, containing specific data from thousands of records (manually verified), and the model followed the instructions perfectly (yes, I know this isn’t what it’s meant for, but I created an environment where it worked fine, even if it was a bit of a compromise).
However, since yesterday, it’s behaving like it’s had a stroke – it doesn’t follow instructions, it’s sluggish, the answers are short and very superficial, they don’t reference any data, and instead of processing files fully (small, 250KB or less), it’s doing analyses based on only 25 records (very small samples). To make matters worse, it doesn’t correct errors – it gets stuck in loops and can’t get out.
I’ve experienced this before after an update – should I understand that this is happening again? And does that mean every update will reset the mechanisms of cooperation I’ve worked so hard to establish with the model, resetting it to the intelligence level of a stone?
I’ve seen other threads discussing this topic… Maybe it’s high time something is done about it?