Phal with spots on underside of leaf (top seems OK)

[cythaenopsis]

This plant had some root rot that was cut off and has been growing new roots. It actually seems to be quite active at that, with a couple more new root tips appearing on the lower crown. It also had a fungus gnat infestation that was recently purged.

What might be the cause of these kinds of spots? They're actually subtle when looked at with direct light. In this photo, they're enhanced by the light coming in from behind. The top part of the leaf doesn't show these spots.