It's because speaking with a muted mic isn't the same as talking to someone. It's confusing - but you have to get used to thinking about it in terms of "performance", we could say. If you're just on your own, that's a "solo" performance. As soon as you're with others in some capacity, that's a "social" performance, and that's where stutter tends to be a thing. There are variations even to this, and that's also something we have to learn to expect. That's how fickle stutter can be. :)