A lightning wallet shouldn't have issue to make transactions during on-chain congestions because that's exactly why they are made for: be able to make instant transactions with very low fees without carrying of the on-chain network congestion/high fees. But muun is not a full lightning wallet, they make submarine swaps, it's more like a on demand lightning service, so when bitcoin network is congested is unusable  

 Thanks for the explanation amigo. I had heard about submarine swaps but never fully understood them.