It's a bit of both I'm sure, but not just limited to Mutiny. Lightning inherently has fees when opening and closing channels, especially when it's a force close. Mutiny or any other self custodial lightning wallet will occasionally have issues. I've seen quite a few people post over the months about channel closures with Mutiny specifically, but that could also just be because a lot of people here use it because of its integrations with Nostr.