When I try to add a border (Filter>Decor>Add Border) using GIMP 2.10 the errors shown in the attached file appear. While it leaves the expected border un-drawn it does seem to leave the added border area selected in such a state that simply using menu item "Edit>Fill with FG color" seems to have the effect of finishing the job.
Yet another bug to add to the list. Was working in Gimp 2.9.9 but not in Gimp 2.10 At least there is a work-around at the moment.
Been looking at some old scripts/plugins for making frames. Needless to say my favourite FrameFactory crashes mightly. If you get gmic going there are Fames in that.