Did you recently update OBS? I'm several updates behind on OBS because I'm personally not yet ready to update all of my extensions/plug-ins/unsure if they all have been updated to match newer versions of OBS.
Some steps I recommend would be:
> Deleting the source and re-adding it brand new to both scenes you want to be able to have them in.
> Double checking visibility in the Sources.
> Checking compatibility with different plug-ins if you're using any (whether they aren't updated, etc? Though I don't think that would really effect PngTuberPlus's functionality, but just in case. Like, removing one at a time & keeping track, seeing if eventually one of those was interfering?).
> Maybe try making the PngTuber Plus Source a Nested Scene and using that Nested Scene in your two scenes?