Audio/video out of sync after demux-remux

[CarryNL]

Hi all,

I've been adding subtitles to dvds for a while now, using method 2 from this tutorial: https://forum.videohelp.com/threads/277950-How-to-add-new-subtitles-to-an-existing-DVD and it hasn't failed until today.
I did everything as I used to, but I found the remuxed result was out of sync. The audio was about 2 seconds too fast.
So I did everything again, same result.

I then demuxed just one episode (I'm adding subs to a 4 episode DVD) and immediately remuxed it, without doing anything to it.
Again the audio was too fast... or rather: the video started too slow! I noticed about 2 seconds of black screen while the audio already started (as in the original). During those two seconds my player (not set to fullscreen) changed size from widescreen to 4:3 back to widescreen and after that the video started. In the original the video starts immediately.

I've never seen this before... and I can't figure out what to do to prevent it from happening
It's probably something easy like a setting I overlooked or something but I can't find it... any guru around to help out this helpless amateur?

Thanks!
Carry.