For some reason external subtitles (subrip/srt.extension) seem to gradually go out of sync as (90+ minute) movie progresses.

At first it's barely noticeable, but around halfway through and during the second half it get super annoying. I've never seen this happen before. I have, so far, confirmed it on the last 4-5 movies I've tried. On the PC the subs are 100% in sync.

Has anybody else experienced this?