https://www.vegascreativesoftware.info/us/forum/subtitles-not-playing-properly--88534/

We even gave you the solution:

The normal command line after a subtitles end time is reached should be (as outputted by other professional software like DVD Studio Pro and Scenarist):

Set field indexes
Stop

 

DVD Architect uses the following command chain in the muxed disc:

Set field indexes
Stop
Set field indexes   

Not complicated to fix.

Thanks.