I've installed the snapshot version of handbrake from handbrake.fr directly (via flatpak)
and verified that on the same DVDs, it generates the subtitles perfectly.
I also built the debian version of handbrake from source and that one still has the same problems.
One can also see the subtitles overlaying each other so that in rapid dialog multiple subtitles appear
one on top of each other, making them all illegible. This does not happen in the handbrake snaphshot.
thanks,
--jack