It's probably due to lack of Co2. You could try starting to dose liquid co2 if you want to keep in low tech.
There are 2 out there, easy carbon and flourish excel. Sounds a like an imbalance in nutrients.
However if it is a new plant melting is common while it gets used to your water parameters.
I'm quite new to planted tank and currently running a low tech now for 4 weeks and the plants are doing excellent. I have 2 swords and another few stem plants which I don't know the name of and no melting as of yet, I have had a few of the old leaves drop off but the new leaves are growing okay!
Do some research into Co2, using an air stone could help with the gas exchange however bring co2 into the tank