What’s wrong

defoliated it so you have a ton of immatre, useless tiny leaves? Dont defoliate. Leaves are good. leaf symptoms are indiscete, so knowing where it started on the plant helps eliminate possibilities.. immobile, mobile, things somewhere between etc, it's not always binary but more of a spectrum. when it progresses around the serated tips first, often relates to low K. Potential nutrient issues from giving too little/too much are also exacerbated by defoliation. LEaves are a buffer of stored nutes and a starge facility when providing too much - up to a point. When you cut them off, you are just throwing out large portions of fertilzier you fed to the plant over the previous many weeks.

Impossible to answer accurately without a diary and not knowing what you have been or have not been doing. Could be a issue related to a build up of nutrient salts in the substrate, especially if you have not been watering to run off each and every time you irrigate..........but without a diary..............who really knows??

It might be nothing. Where are these leaves growing? If it’s way under the canopy, it could be from inadequate lighting. Could be a deficiency as well. But if you’re not seeing large/older fan leaves showing the same issues it’s most likely NOT a deficiency. If it’s under the canopy it’s likely due to inadequate lighting. Look for signs of pests. Always be diligent of pests.

Maybe a buildup??? no diary = no info so no real idea.