Looks like some sort of chemical burn. Did you spray the with something?

Nope. I mist them with RO filtered water once a week, that's it.

Too close to a hot light? Or, by any chance are you watering with water from a water softener?

The grow lights above them are LED (square panels) and they make pretty much zero heat.

I water with city water run through a reverse osmosis filter. I'm not sure about the hardness of the water. I did check soil PH with a meter a month or two ago and it was normal.

A dog, a cat, a bird?

spray it with a garden fungicide and lets see if that helps

There are no critters near the plants at all.

I've never used a fungicide, I will give that a try! Anything in particular recommended for citrus? Or will any fungicide work? Thank you everyone who has responded. I appreciate the ideas.

I bought some fungicide and figured I'd mist the leaves with it until they were saturated. It turned out to be thick goopy stuff that splattered on and beaded up and ran right off the leaves! So, I poured some of it in my spray bottle, added a little water to thin it slightly and added a few drops of Dawn dish soap and got what I was after. No further browning so far. Hope that was it! Someone also suggested sunburn and indeed my LEDs were pretty close. I gave them some additional distance.

I'm loving the forum here and am learning a lot. I read a thread about fish emulsion (which I had never heard of). I bought some F.E. (5-1-1), misted my leaves with a diluted mixture and watered the trees with what was left over from my mix. Both my lemon and orange greened up tremendously overnight. Amazing.

Read another thread about heat mats and someone talking about the desired temps of the roots. I have mats that I used to get these trees going but turned them off months ago. After reading about the target temperatures, I realized my house AC keeps things on the cool side for them. Heat mats back on!

Thanks everyone.