FCP X master file export messes up embedded captions

Hello

I'm editing on FCP X 10.4.6, I have a 25 minute documentary with CEA-608 type captions covering most of the video (I'll use them as CC-subtitles later on YouTube or Vimeo).

When I put an in and out point covering a random 1 minute selection with captions and export that as a Master File, the resulting exported MOV shows the embedded captions without a problem and perfectly in sync in QuickTime Player.

But when I choose to export the entire video, for some reason all the captions end up in different places in the MOV, or don't show up at all.

How can I fix this problem?

Thanks for any help!

Sincerely

Alexander

Belgium