I've been seeing the behavior for several weeks now. You can tell because I've been sprinkling my own posts with triple and quadruple spaces between sentences. I see it in my own posts, and I see it in others' too. What may be especially interesting is that if I reply to someone via Quote, I do see normal spacing in their text while I'm in the editor. When I finish and click Post, the lack of spacing is noticeable again. So I wouldn't phrase it as the system removing spaces, merely failing to display them. I'm using Firefox 36.0. I see it also with IE 11.0.9600.176341. I'm on Windows 8.1, in case that matters; in fact I'm leaning toward thinking that it does, given your difficulty in reproducing the behavior.