Really appreciate you taking the time to look at this. I know dvd isn't the most popular format anymore.
For science I just tried 25 different dvds, all various sources, some iso, some video_ts, some NTSC, some PAL.
Every single one used libmpeg2, so I was having the opposite luck.
The only one that played back without "bad" deinterlacing is one that is progressive film encoded at 23.976fps with pulldown applied, so of course in this case there is nothing to actually deinterlace. It still used libmpeg2 but this one displayed the framerate as 59.94 for whatever reason. Everything else displayed 29.97fps or 25fps in the playback statistics.
The one I tried yesterday that has no menus does use ff-mpeg2video though.