Did you check that you don't have the hotkey created twice? Maybe even in different hotkey profiles.
If you have the hotkey in profile 1 (active on tab 1) and also in profile 2 (which is active on tab 2), it would trigger the action twice, once from profile 1 on tab 1 and then again from profile 2 on tab 2. Since the action changes the mute state on all tabs, it will end up doing what you describe in that it toggles the mute state to muted, and then back to unmuted from the second action.
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