I'm going to give what I think about what is causing it.
Basically it's a plugin. A mute plugin to stop people from overtalking the warden. Have you ever noticed, that if the 'glitched' muted player becomes warden or gets !forcewarden that suddenly there mic is unmuted...And after they've been unmuted they can now talk on red team with no mute at all.
Sourceban muting and unmuting also sorts this, but it isn't what is causing this. It's how this plugin is working. If you choose to remove this plugin though, you'll get a lot more overtalkers.
Sometimes though the plugin doesn't work and let's people talk without the mute. You noticed in the chat where it goes 'Red team has been unmuted' and thing like that? That is the plugin at work.
Basically it would be worth finding out which plugin is causing this, or command to stop it from doing this. Though it all depends. We might be able to find a way where it only takes a couple of commands to stop it from glitching. Maybe just a plugin update. We needs research!