It's not something I recognize, but I don't know that many. :) Where did it come from?

Also, are we seeing full shade leaf color, or has it seen some sun?

Certainly it could be a seedling.....can you see the graft union?

I am not inclined to ID specific cultivars of Japanese maples. While I have grown and sold scores of different types over the years, there are simply too many different cultivars available with minor distinctions to separate them to address the subject with any authority. And the ever-present possibility of a tree being seedling grown, in which case there is no ID beyond the straight species.

One better online grower expresses it this way : "There are things that are absolute in life such as death and taxes.
Unfortunately being able to visually identify a specific Japanese maple
cultivar with 100% certainty isn’t one of them. There are several
reasons for this. First, your tree may be seed-grown guaranteeing that
it is not a named cultivar. Many older trees that may have been grafted
lose there graft “markings so it is impossible to tell whether they are
seed grown or not. Second, many Japanese maples are similar to each
other in color, leaf type and many other visible ways. The actual
differences in many cultivars can be very subtle. Even with the ones
that are VERY distinctive you cannot guarantee an ID although you could
give a “likely” ID. There are also many differences brought about by
environmental conditions that could also make the actual subtle
differences in appearance even more obscure. Finally, the number of
different cultivars that have been produced over the years is
astonishing and may possibly number a thousand. A person would be hard
pressed to know all of them in any intimate way. In my opinion it be
unethical for even the most expert old-time Japanese maple grower or
even J.D. Vertrees himself to give a 100% guaranteed ID on any Japanese
maple."

This is the shade colors. It gets some morning sun but not a lot. I looked for a graft union but didn't see anything that would strongly indicate where it was grafted. For the record I have several other maples of the same size that I know are grafted and the trunks looked the same so I am not sure how much of an indicator that would be.

As a follow-up question to seedlings, do seedlings follow the parent tree as far as general size at maturity?

Identification of a graft union just removes the possibility that the tree is seedling grown. All named forms will be grafted - any seedlings, regardless of parentage, are just generic species Acer palmatum.

Seedlings may or may not resemble the parent plant(s) in habit, size, leaf shape or color. Typically, at least a few seedlings may very closely resemble their parent but many more may be very different. Until you grow them on a few years, it's difficult to tell for sure what they will look like or how large they will get.