Subtitles not synced

My movie and subtitles play perfectly on my NAS and are in perfect sync. However, when I play this exact item on Plex, the subtitles are extremely out of sync. Why, and how do I fix this?

Is it transcoding when playing through plex?

Hi Guys, I have the same issue, subtitles that are perfect when I play the media via my pc are offset (3000 MS or more) when playing via plex. Does not matter whether I play via my iphone or appletv or even my browser. Quality is shown as original (so I assume no transcoding ?)

I did not have this issue before, seems to have arrived in the last two months or so. Any suggestions on where to look and fix?

Thanks,
Patrick