Muun is selling itself as a Lightning wallet, but it actually ain't. They are doing (submarine asfaik) swaps in the background and your btc are stored on-chain. That's ok, but when the onchain fees rise, the fees for their payments rise too. So you might have small amounts in your Muun, but you can't use is as the fees are higher than your balance. (maybe that has changed recently with an update, I don't know)