When pressing the hardware keys on my laptop for brightness and volume, the volume and brightness change but the on screen display is not shown. The output display using Meta+P does show.
Interesting, cannot reproduce in my built-from-source session, and a Neon person with an installed-from-packages session also could not reproduce it.
This problem started for me at the same time this issue was created, I'm on OpenSuse Tumbleweed with Nvidia proprietary drivers and X11. I have noticed something interesting, when you press multiple time the volume up button at some point the GUI show up. It appear that when you increase the volume and it reach 100% the GUI appear and the volume goes to what the GUI shows, the same goes in the other way when you decrease the volume and it reach 0% the next volume down will make the GUI appear and the "real" volume will be set to what is displayed. Also when the volume is muted ( GUI show it muted ) and you press multiple time the volume up button you will have no sound until you reach 100%, only then the GUI will show up and the sound will come back but it will display 5% while the "real" volume will be at 100%.
The problem seems to be fixed now, my audio volume keys are working as intended again. I have no idea what caused it in the first place.
I'm guessing a change in NVIDIA or even Xorg fixed it.
I just started experiencing this behavior after updating to 5.27.9 on Debian Testing; no issues with 5.27.8. FWIW, my iGPU is from AMD and I'm on Wayland.
Also, increasing the volume to 100% does not enable the OSD.
For what it's worth, this just started working again after sleeping and waking; would love to know a good way to troubleshoot this further.