@DaisyDo. My pots have underground water. They sit inside a wet/ or almost pond situation. From what I've read they take a rest after bloom. They do dislike permanent inside water condition. In Japan ( from what I've watched) they are grown inside large flattish containers that they move around. They aren't placed in permanent pond environment. In winter they should be kept above the water line. They seem to withstand frost easily if kept on the drier side then. The brims of the pots can be seen easily in your lovely video that you shared!
I just found this AIS webinar video:
https://www.youtube.com/watch?...
as of 51: onwards there is quite a few tid-bits about growing them. btw. this webinar is very recent, uploaded only ten days ago.
Perhaps what you have read/witnessed is the effect of a toxic chemical that has to be removed every 3 years if left in ground or in pots without change of soil. This I just learnt from the video.
So the solution to my seedlings is frequent repotting! with plenty of well aged barn manure ( almost half of the pot) of which I collect daily in huge quantities
Well like in every garden endeavour there are dots that one needs to connect to get it right. This has really made my day and just want to

because I was able to make progress...
Arturo
PS. Just went out and checked all of my ensatas! Most are doing great. Chad Harris suggests to repot them every year. So that's exactly what I did. I did add a generous 2" on top of barn manure for the few that were already in larger grow bags. They are now happy sitting in a puddle...