You've kind of answered your own question :-))

Yes, growing an Acer palmatum under lights in winter and at temperatures above what the tree would normally experience outdoors is most likely prompting the coloration. These trees need to be dormant in winter. While they can survive for awhile without experiencing a proper dormancy, it is a very temporary situation that I would not be inclined to prolong. Without a routine and very necessary period of dormancy, the tree will decline and eventually die.

The coloration you are experiencing is most likely due to the concentrations of pigments one normally sees in deciduous trees as days shorten and temps cool......IOW, the tree is getting ready to enter dormancy.

FWIW, light in winter is immaterial to a deciduous tree or shrub that is allowed to enter a proper period of dormancy. What you need to home in on is proper temps (no higher than about 45-50F for at least 12 weeks) and a reasonable level of humidity.