Hello!
Had the same problem on my laptop (Arch Linux, niri compositor, DMS 1.4.6, PipeWire, Cider 3.1.8).
After Cider installation on a brand new system (PC with the same: Arch Linux, niri compositor, DMS 1.4.6, PipeWire, Cider 3.1.8) found out, that media keys are working and DMS music widget is also working.
Then, I made a simple research:
Problem: playerctl --list-all returns empty — media keys, widgets, and MPRIS integration don't work.
Cause: ozonePlatform set to auto or wayland in Cider's client options breaks its MPRIS D-Bus registration.
Fix: Open the file $HOME/.config/sh.cider.genten/client-options.yml in a text editor.
Before (broken):
linux: useMpris: true gtkVersion: "3" ozonePlatform: auto
After (working):
linux: gtkVersion: "3" ozonePlatform: x11
Then, Restart Cider.
Hope this will help someone.