Nitrogen deficiency?

At first, I wouldn’t worry too much since it’s only on one leaf and the rest of the plant looks fine. If the issue continues to spread, I’d recommend checking your pH and making sure you’re not overwatering. If both of those are in check, then it might be a magnesium deficiency.

Hey there sense you are doing bottom feeding I garentee your starting to have a build up in the soil causing lock out. Water from top and check run off waters ph it will be low then run a few Gallons of tap alwater thru the soil to bring ph back up and wash away some of the salt build up in the soil

It may progress so I'd suggest upping your base a little bit. Another 25%. If you find it's interveinal and the veins stay green then it's prob a mag issue. She's just hungry I think. I've not looked at what you are feeding

Doesn't look like serious nitrogen deficiency. One old fan leaf yellowing can happen naturally. Your pH and EC are fine. Just monitor if new growth stays healthy, no action needed for now!