Why doesn’t my 3rd party keyboard control volume in macOS 15.4?

Given the hardware you’ve already tested across Intel and Apple Silicon machines, it’s fairly clear that this isn’t an architecture-specific issue. The fact that everything worked fine under 15.3 reinforces that 15.4 introduced a change in the way macOS handles HID devices — probably tied to Apple's tightening of input driver support.

I'd suggest filing a feedback report via Feedback Assistant with specifics on the devices and macOS builds — Apple typically won’t acknowledge this kind of regression unless they get enough noise.

If you're up for it, monitoring IOKit logs via Console.app while pressing those non-working keys might help narrow down whether macOS is seeing the input at all. This would require enabling HID debug logging with the Console.