Yeah, technically not a bug. If you look at the HTML for the thread, the whitespace is actually there in the source. It's just that your browser is stripping the extra spaces because it isn't supposed to display more than 1 consecutive space. Tooltips do not obey this rule, hence the discrepancy. This is really the browser's fault more than anything else.
It could be fixed by stripping whitespace from thread titles, but that's not ideal.