Same problem here.
What I found out is that the mute button seems to work but mutes wrong tabs. Might also be trying to mute non existing tabs as sometimes I can’t find one that is muted.
The problem seems to occur mostly while using several workspaces. I also had the problem between tabs in a single workspace but couldn’t replicate it.
Replicating the problem between workspaces is easy. I just open tabs in workspace 1 and in workspace 2. While I can mute the tabs in workspace 1 the tabs in workspace 2 either mute a tab in workspace 1 or no tab at all.
Opera GX LVL4 (core: 98.0.4759.56)
Update stream: Early Access
System: Windows 11 64-bit
Chromium version: 112.0.5615.165
Windows 11 Pro 22H2 (22621.1702)