I think it would really help if you can share your tank parameters, high light? CO2? dosing schedule? heavily stocked
I think the lighting and CO2 may not be strong enough to keep the old growth healthy. I'm sure the tops look great but then it pulls nutrients from the old growth.
Do you have soft water? hard water?
I have also noticed that even though I had hard water I may have low Mg. Since dosing Mg separately from my micros I have been able to avoid the pin holes in the old growth.