I haven’t seen anything like this before.
Now this is a different type of tank (low tech no CO2, EI once a week, and poop machine breeding fish) Some times my planted cichlid tank (electric blue acaras and severums) gets funky and plants start to randomly turn to mush or drop all the leaves.
What seems to turn it around is daily huge water changes for 3-4 days some times a week. I do two back to back 50% changes before starting to do the full tank drain to acclimate the tank sort of slowly to the big water changes. I drain down until the fish all but fall over and refill. I try to vacuum the sand and the sponge filters as deep as I can with every big water change. Redose the macros and GH with every water change for the full tank volume. I can’t ever pinpoint what causes the melt because sometimes the substrate and sponges are funky other times it’s not bad.
The only constant is the multiple huge water changes and cleaning turns things around. I am not saying this will fix it just that it helps me get out situations where my plants are going south and I can’t explain why. It might help and definitely won’t hurt.