Perhaps not everyone would agree with this finding, but I'm coming near to the conclusion that when you started adding KNO3 and KH2PO4 in a high growth tank (high light, high CO2) and if every other plants start doing well except lotus and/or crypto having melt-down/blow holes/rot edge, it is a sign that the tank is running low of Ca/Mg relative to the NPK.
I can't tell if it was Ca or Mg alone, as I tried address them seperately but with no effect. But a combination of ratio 3 Ca to 1 Mg (Ca 6 ppm, Mg 2ppm, twice a week) seem to bring some positive response.
I add Seachem Reef Calcium (which is Calcium Gluconate) and Epsom salt.

Alternatively, adding Seachem Equilibrium will fix the Ca+Mg issue too, despite the abundant K that came with it.

I believe not every tank may face this could be :
- their tap water already loaded with enough Ca+Mg, or
- their substrate (or base fert additive) has more Ca+Mg, or
- they feed their fish with Ca+Mg enriched food, or...

Sometime we though it was the new cryp or lotus that needs time to settle in, but do we see it takes much longer to settle in a high CO2 tank than in a non-CO2 tank ? I suspect that in a high light, high CO2, high 'NPK' tank the demand for Ca+Mg is higher but the new plant had not establish their root deep enough to harvest the Ca+Mg in the substrate while water column Ca+Mg is depleting.
Hence the meltdown happen.