Ubuntu – MultiMedia Keys not Recognized in Custom Shortcuts

I'm in the process of getting 20.04 setup after having upgraded from 19.04, and part of it is getting my MM keys to work with spotify. While trying to make the custom shortcuts, I have run into a problem. On pre-existing system shortcuts, the MM keys on my keyboard are registered as valid inputs, and can be assigned:

However while trying to assign a keybind to a custom shortcut, the keypress is not recognized, and I am left on this screen:

What I have tried so far:

• As suggested in the comments, followed this post's suggestion of disabling default play/pause keys from dconf-editor. No change.
• I tried with a second keyboard (also with media keys), to no avail
• I tried to bind a key and unbind it, also no help
• Looked around in gnome-tweaks in case something got added/updated but couldn't find anything.
• Came across this post, however, the solution there now shoots an error:

david@david-pc:~$ gsettings set org.gnome.settings-daemon.plugins.media-keys previous ''
expected value:
  (empty input)
  ^

• After updating to kernel 5.4.0-28-generic, the media keys worked briefly, then seemingly randomly stopped again. After rebooting, the media keys started working again, then promptly stopped working after around 15 minutes.

Is this a bug, or am I doing something wrong? Either way, I would love an answer/workaround.