i thought/read the same thing about UG not liking Nitrates until Tom Barr debunked it.
Ferts for UG
I have read that UG doesnt like DSM as well.
I bought $100 worth of UG that i was told was grown emersed. I planted it DSM, within a week i had lost over 1/2 of it to melting. I flooded my tank and the melting slowed and eventually stopped.
Hopefully yours is just adjusting to the flood like mine did