What’s your fertilizing and flushing routine?

It may be possible that you don’t have a “real” deficiency, but a nutrient lockout due to wrong pH or imbalance of salts.
The lower leafes are most affected, so it seems like a deficiency of mobile nutrients like nitrogen or magnesium for example. The leafes are also drooping, which might indicate an oxygen defiency (overwatering), as you already mentioned.

Did you measure the EC or pH? What did it tell?

How does you setup look like? Does the plant stand in a cache pot, or directly in the outer white one? How high is your nutrient solution level usually? Do you use a water level indicator?

And where is it located? Does it receive enough light?

Removing the rotten parts that radically was a great idea :)