What’s wrong

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.