Yikes. Muun works as a Lightning wallet but it's not a true Lightning wallet. These fees will happen sometimes as they swap from on-chain to Lightning and vice versa. Look into switching to Phoenix or Mutiny.
Discussion
So I’m using Phoenix for most transactions but had been using Muun for certain things (think certain Lightning network payment bonds which usually fail and return to the wallet…)
Up until the past week, those failed payments would return in full. Now they keep the fee 😑
Done with Muun, I think, except for when their submarine swaps are cheaper than Phoenix’s swaps go on-chain