When does this occur constantly?
If i open “Find in Files” (via Ctrl+Shift+F, or via plugin’s toolbutton) and do a “Search all”.
A docked window keeping the search result(s) will open - and the toolbar ‘disappears’: it appears to be empty - only the panel is shown, without buttons.

Will the toolbar not be drawn at all?
Not quite: if i move the mouse over a toolbutton, this single toolbutton will be (re-)painted.

Will this never end before a next restart?
Basically not. Unless i minimize/restore the NotePad++ window, or force a redraw of the docking form by moving the vertical splitter.
Then the drawing is back again.
But - if the search result window is still open at that time and i close it now, then i’ll have an empty toolbar again.

Is such scenario already known somehow, and are there belonging notifications to reactivate the drawing?
Does a notification exist that informs about the appearance or disappearance of such docked subdialogs?
(If i’d know this i could enforce a repaint myself).