There is an option for music widget to open the app in the PiP not full screen.
However, if you chose Tidal, PiP option is available, if you choose General Music, it isn't, if you choose Neutron Player, it is, UAPP isn't, YT Music is. 


And so on. This is completely independent from PiP settings.
Why some apps are available, and some not?
Generic Music would be preferred to have that PIP option.