I haven't been able to reproduce this, so yeah another video would be good.
That said, I'm unsure how much I can do about this as the browser is generally responsible for tracking its state here, so if it gets confused and bolds something unexpectedly, it's likely because it didn't reset the state when the cursor moved.
I've confirmed the behavior - it seems to be related to hiding the container that contains the editable area. A workaround may be possible, though it would need extensive testing on other browsers. Alternatively, it may just be easier to report the bug to Mozilla (with a reduced test case), which is probably what I'll look into.