I used to be able to highlight a piece of a post and then hit quote selected and it would be in my post. For ages it no longer works and it just puts the whole post in the reply box. It's a pain when trying to answer multiple questions on sleep threads for example.
I have the "quote (selected)" button too. I always found that it worked for almost every thread but when replying to certain members it never worked and would quote the entire post, yes very annoying. Even now the same members come up with the entire post quoted instead of the selected area but as I don't mod now it isn't really a big deal to me.
Sorry, didn't help to fix it, but maybe you notice it is certain members and not all? It could lead to finding a reason why it happens sometimes?