Sounds weird, but CO2.
I've seen this with Java fern and a few other plants, they melt in response to CO2.
Having had chronic CO2 issues on a number of client's tanks over the years really tells you if things are going right, if they add the ferts 2-3x a week, and they say so, it's very likely that they did.
But that just leaves the CO2 and when the rate is dropped and they have a low sump water etc, or have reduced it up because they think the fish are having issues with the CO2, that's when I see lots of CO2 related issues.
Hygros, Java fern, Bolbitus in particular seem to do this.
Regards,
Tom Barr