Will a lack of CO2 cause stunting like this?
I am no expert
Inter stem elongation well if this was a High Tech tank with CO2 injection @ceg4048 would be quoting the experiments done on Rice and saying it was due to poor CO2 levels with poor flow in the tank. So it would follow IMO that in a non injected CO2 tank the Inter stem elongation is due to the same poor CO2 levels plus lack of flow would still apply as the experiments done on rice wasn't with CO2 injection and was based on the CO2 levels at certain depths in the rice paddy fields with low flow. The rotala that is growing well with shorter distance between the stems is getting better flow and hence CO2 so the distance is shorter plus getting more light so the rotala is also growing better. Flow may also be better higher up in the tank.
I see this same picture on my Rotala in my potted tank high tech tank, when the rotala in pots is planted close together the rotala has small leaves as it competes for the CO2 and because its so closely packed together flow is reduced which compounds the CO2 levels again the result is Inter stem elongation and small leaves, a few rotata break though which then get better CO2 levels and better light and presso the poor growth transforms like your has plus the leaves turn reddish dependant on light levels. The only difference is my tank is CO2 injected and yours isnt but IMO the cause is the same CO2 deficiency due to inadequate flow.
My rotala has the veining on the leaves just like yours not quite as distinct.
Plus your P. Pogostemon helferi shows no signs of an Fe issue IMO and its Pogostemon helferi thats show Fe issues first in my tank with hard water.
Think the tank just needs some CO2 injection 1-3 BPS 24/7 would probably work wonders with tad more flow OFC