I used the beta 2.8.91. Closed it, but it was still resident in memory using 366,8 MB. I had to use the task manager to close it.

I am on Windows 8.1...

It's possible that that happens under some circumstances, for instance when Krita was still computing an action while closing. It seems to be quite rare and is _very_ difficult to find a fix for, though we're looking.