mp.set_osd_ass() (which was undocumented, or in other words, was not supposed to be used by external scripts) used to do change detection in the mpv C code. If the resolution or payload did not change, it was not re-rendered on the lower levels. Apparently this made some people sad, so fix it. (But only after I told them to fuck off.) (Well I didn't put it this way, but still.)