Bet the one I sent... it was all grown immersed, btw.
My first guess it's trying to adjust to you water condition. Is your water hard or soft?
Then, how did you plant it? Mine usually have pretty long roots. When I replant it, I cut the roots with sharp scissors to about 1/2" to 1" from the leaves, leaving just enough so that it would stay in the substrate. Plant it a bit deep and then pull the plant so that the base of the roots is just visible above the substrate,
It does like a bit of light, so, I would plant it in the open spot and maybe 1/2" or more apart.
Are all the leaves melting or mostly the outside ones? I would peel the outside melting leaves. A plant with 2-3 leaves will still survive.
That's pretty much my brain dump at the moment.
How are the other plants doing? If some others are doing ok and some others are not, let me know which. That might give us more clues.
And a bit of luck does not hurt either