I can confirm that the behaviour as you describe it here happens on my setup as well, however I have a couple of points to add.
Keep in mind I'm not a coder and I may be way off-base here; I may also be totally misunderstanding what you are saying, but if I was writing a style sheet (as an aside it makes no difference if it is saved or not or in the project folder or not), I would use the way you have written your CSS statement more for an inline style rather than for use in a stand-alone style sheet. When I create a style sheet, for clarity I put everything on a different line, which again I stress I am not saying is the absolute right way to do it, it is just the way I like to do it. Like I said, I wouldn't know if this is correct or not, it is just my opinion. However, in support of this, doing things this way also makes more sense because of the way WeBuilder formats it in the inspector. It doesn't put every rule on the same line (except in the main heading for each selector) because it has a separate line for each rule and each rule is assigned its own line. Refer to my image below; it is easy to see background-repeat is set to no repeat; background position is bottom and background image is centred. If you look at the background selector heading though, you'll see everything is included in that line, there just isn't room to see all of it unless you drag the borders out like I have here for the sake of clarity.
I agree that it is not that useful to have autocomplete not available once the selector and rule has been written, however, if you put a ; after each line and begin a new selector and rule on another line, the autocomplete does work again, which I am guessing is how it was intended to work in WeBuilder with style sheets.
Of course, I might also be full of it and not know what I am talking about so just ignore me if that's the case - it is the only explanation I can think of to explain the behaviour of the editor.
Dave
davethompson.co.nz