Is this a heat from lights issue?

I checked these plants about 3 hours ago and they looked perfect. Just went in and two of them had this brown discoloration. They were transplanted yesterday with no issues into 1 gallon pots from their seedling containers. They are just shy of three weeks old. No nutes, just plain rainwater added to soil. MH 400 is 10" above plants and passed 'hand
test.' I raised it another inch. If it was a heat issue I would expect to see the newest growth at the very top to suffer first and most. Or could this be a reflected light issue? Pics attached.
thanks,

Bra

edit - this discoloration is only on topside of leaf....
 

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