Mediaconvert not adding some CEA-608 captions

I'm using mediaconvert to embed CEA-608 captions into a .MP4 file. It mainly works - the output file contains most of the captions - but occasionally it drops a caption and doesn't embed it.

Does anybody have any idea why, or how to troubleshoot this? All the captions are of under 32 character line lengths and so on.

Thanks!