I think I've found a bug in Sync/Convert.
When converting media with embedded subtitles:
If the user who created the convert job has their playback preference set to Always Play Subtitles
The process works properly extracting a valid vtt file.
If the user who created the convert job has their playback preference set to Only Forced Subtitles (and I bet anything that in that particular instance wouldn't trigger subtitle display):
The process creates a blank vtt file.
I've worked around this by creating a dedicated user with Always Play Subtitles, then logging in as that user just to create convert jobs.