I haven't noticed it.... and I have 300+ categories.
However, I have noticed that if your BuyNow module is on the same page as your category module, do NOT put the name of the page in the bottom of the Categories module. This does 2 things; 1) It keeps the categories expanding properly, especially if you style the categories like I have, because the module is then communicating using inter-module communications instead of post-backs, and 2) it keeps it from doing a post-back. As soon as you do a post-back, it collapses the categories back to default again.
Am I rambling? I'm sorry... I have a horrible cold.... I hope I'm making sense today. lol