Leaf

Can't see shit on pictures taken under grow light! Always post pictures taken in "normal" white light. If it is affecting the top leaves first it is usually a trace element issue. A dose or two of soluble trace elements available from any garden centre should clear it up quite quickly. But really can't see much, so this is a best guess answer.

Hard to tell is it a rusty orange or a narcotic brown. If it's narcotic then it could be a boron def. If it's more orange the possibility excess manganese (not magnesium). Both are micro nutrients. What are you currently feeding them?

If its only the very top top leaves its light/heat burn from ya lights. Lift ya lights up a bit higher if possible or get a fan/ get a better stronger fan.

can you post a pic without the lights on too hard to see in that orange glow