I have a few videos which are 1080p, with VobSub subtitles extracted from the DVD.
In VLC or IINA, the subtitles are displayed in the bottom center of the screen, as they should.
In Infuse (I tried on Mac and Apple TV), they appear at the bottom center of the top left quarter, and are not scaled up. There is no option available (that I could find) to adjust the size and position of those VobSub subtitles.
(I really want to use those because it’s from a not so common PAL DVD release of a TV show - I slowed it from 25fps to 23.976fps and upscaled it to 1080p)
I can send a sample file if that helps.