To reproduce this, one has to mute their microphone (if it hasn't been muted already) inside of Wiremix, then restart their graphical session. After opening Wiremix once more, you'll notice that the microphone is still picking up sounds via the volume indicator, even though it says that it's muted. Unmuting and muting the microphone again temporarily fixes the problem.
I've experienced the same problem with pwvucontrol, so maybe the cause of this issue lies beyond Wiremix. I haven't experienced this issue with pavucontrol though.
To reproduce this, one has to mute their microphone (if it hasn't been muted already) inside of Wiremix, then restart their graphical session. After opening Wiremix once more, you'll notice that the microphone is still picking up sounds via the volume indicator, even though it says that it's muted. Unmuting and muting the microphone again temporarily fixes the problem.
I've experienced the same problem with pwvucontrol, so maybe the cause of this issue lies beyond Wiremix. I haven't experienced this issue with pavucontrol though.