Perhaps more traces(and not just Fe) and when you add the NO3, PO4, K+, even GH, this puts a larger demand on the traces.
Still, folks would have seen this more before this. But few folks had ES back then, now everyone has it.
It grows well for me, at 20ppm but I have not tried high K+ on it but I know many that have. It's a funny plant but some of the nicer looking plants have been raised in higher K+ levels.
My advice would be to add more NO3, more traces, maybe some Ca/Mg and then see if the high K+ has much effect.
I suspect it will not.
I think some folks have not been keeping some other nutrients, parameter etc in line.
Some approach things conservatively, too much, or some go the other way and over dose.
Some mix these two together.
Some things are not talked about such as flow rates, plant biomass, filter size, and other non Nutrient issues and this can account for some variability.
I don't know what those folks are doing, but I know what I am doing and what the tanks I've seen in person exhibit.
But this is neither here nor there, I'm mildly interested if it was a real effect I could duplicate, but it's not, so I'm less inclined to do much further work on it after trying the Ammannia out here recently.
So if folks simply add KNO3 and not add any K2SO4 unless they are getting a lot NO3 from the tap or the fish load/low light etc, then
KNO3 by itself for most of the folks with issues should have no problems.
Main thing is to have some excess K, 10ppm is excess K relative to NO3 at 10ppm.
Ratio may help some but the main issue is keeping these nuitrients from becoming limiting. We dose and do water changes often so th3is ratio does not really mean much until things get down low and become limiting or if you try and run your tank very lean, not a good idea IME/IMO.
Some folks might try to add 3ppm daily of NO3/K+ and 0.3ppm of PO4, while I might add 10ppm NO3 every 3 days etc or some might enrich the substrate with something and add less trace, NO3/PO4 etc.
You can skin this cat a number of ways but if things go to zero for long, the plant will suffer.
Adding a bit more and keeping it richer generally helps, NO3 stunting occurs more often than some folks are considering also I believe, especially those that are trying to get more red colors through NO3 limitations.
I and other have been down that road already.