I have a black maple (Acer nigrum) seedling I grew from seeds I bought from Sheffield's. Although I originally had several good seedlings, several died in a "rabbit incident" (they got eaten) a few days after putting them outside, along with several A. japonicums, A. palmatums, and A. shirasawanums.

This one was right in the middle of a growth flush, so I didn't want to transition it outdoors until the leaves from this flush matured.

Today, I just transplanted it into the smart pot from a Rootmaker cell - the roots looked nice and white. I noticed that the two lowest leaves of the current growth flush are starting to look a bit dry/necrotic. In the pic below, if you look at the lowest new leaf (the reddish looking ones), there's a spot there on the right side, near the sinus between the lobes, that looks almost as if it is drying out, or necrotic. It looks a bit whitish in the pic. The opposite leaf on the other side, not visible here, looks similar.

I don't think it's getting too little water. Too much might be an issue, but I let it get just dry between waterings, and it has never wilted. Also, the roots look healthy.

Now I'm concerned the transplanting might do it in, since it was already getting this "dry spot" before I moved it.

Any idea what this is? It almost looks like freeze damage, which would be impossible since the tree is indoors under a grow light.