Question about the memory feture

I am not sure if this is the right place to post this question or not, if it’s in the wrongs spot, I do apologize.
So with that out of the way, onto the question at hand.
Why is it that when the memory gets full GPT gets really dumb?
What I mean by this is that if the memory is full, then if I ask it to do something, it will constantly give me incorrect feed back, or just completely miss the question, and give me something totally random in response to my question.
But if I clear the memory, then it goes back to functioning correctly.
In my mind, the way it should work is that if the memory is full, then there should be no change in its behavior, and just continue on and be like it is when the memory is not full.