Ever since I got my Pixel 4, YouTube Music does not show up as a music provider in the Kustom apps. I assume this has something to do with YouTube Music being pre-installed on the phone as all the other apps, that I've installed, that use playback show up in the "preferred music player" setting. However it could be because of the new YouTube Music update.
This is fixed in the next version 3.47, YT removed media button support which is a STANDARD feature