I noticed the same thing in Tidal at one point, but I believe it was resolved after I re-installed the 8.1 beta, then did both a factory reset and cleared the cache from the debugging screen (with device powered off, hold down power and play button together for about 10 seconds when running android 8.1 beta). After that, I sideloaded Tidal and Neutron again. I just tested Tidal and Neutron, and all media buttons work with screen on or off. I also left the screen off for 10 minutes while music was playing, then checked again, just to make sure it wasn't a wake lock problem. Again, things worked exactly as expected for me. I know it's a pain to wipe everything and try again from a clean image, but that did appear to fix things for me.
@newtophones07: Do check Neutron settings, there is one under Connection and Controls called "Media Buttons" which should be enabled. Tidal does not have anything similar, and historically just works. I'm not aware of any options in the Android OS settings which would affect media button function in 3rd party software.