Does it mean it was working like this before? If so then we'll need a warning somewhere in the release notes (not to be forgotten – should be added to the issue for releasing b2/rc1/1.0 – I think there's an issue already).

Yes, it used to work before, but I think I accidentally committed the filter changes a while ago. This is why Trevor Cox reported this for B1:

2) In the WYSIWYG editor, if I highlight a word and include a leading space (ie " word") and then format as bold, the result will be "* word*" instead of bold text. It works fine if there is no leading text.