Even if Guitarix is working as expected, Tuner is not showing anything at all.
I have tried this on 2 different Linux OSes (Silverblue and Arch Linux) and the issue is consistent on both machines.
the tuner requires setting realtime priorities on its thread which doesn't work inside the sandbox, and it bails out. Guitarix probably told you about the error on startup.
Even if Guitarix is working as expected, Tuner is not showing anything at all. I have tried this on 2 different Linux OSes (Silverblue and Arch Linux) and the issue is consistent on both machines.