All text entered in miniBB is already saved in database converted to HTML chars. I can't confirm your bug, on my side everything works correctly (as such basic operation works for years already, don't you think nobody would notice that for years? ;-)
Most likely you are trying to edit your own topic, but in miniBB, there is a special option which allows/disallows regular users to modify their topic titles. In that case you will simply not see your topic title in the editing form. Look in manual
- it is set to FALSE in default package!