I don’t know if this behaviour has already been reported: PGS subtitles in HDR sometimes look quite dark. Strangely this doesn’t affect all movies, in some it looks fine.
I’ve tested the same file with Infuse and MrMC (see screenshots). Note that the camera makes it brighter in both cases, but you should see there’s a difference. For movies without the “bug”, subtitles look exactly the same in Infuse and MrMC.