Mine is a high Co2 environment with 72 watt light on 15 G tank with ADA AS ii. But the Vivipara are planted in gravel and not on ADA soil. I think they came to me grown emersed, had lots of plant lets on their tips....2nd week after planting, I see that they are turning brown. Why? I have HC Cuba in the same tank and it is thriving.
Could you post your parameters and dosing method? How is it turning brown? Uniformly brown while maintaining structure? Is the whole thing melting? If it's a uniform melt all at once with good parameters, it's probably the adaptation.
I have no experience with this plant. However, with emmersed grown plants they will need a period for their new immersed foliage to grow in. Sometimes they grow in and the old leaves hang around a while. Sometimes the whole plant just seems to sulk and the old foliage sloughs off. Depends on the plant. I've have a sword that did both going immersed -> emmersed -> immersed again.
Its getting brown through out the stem, not melting suddenly. After it gets brown then slowly it becomes soft and gives way from the bottom. It is not in all the plants, many are green while quite a few have melted and the plant lets are floating loose. I think I am seeing new shoots coming out, but I am not sure that it is Vivipara or Parvula as of now.