Yes those beetles are annoying. They usually do more damage to the foliage in my vegetable garden than they do to the roots. I've never seen them on the leaves of my maples.
I'm guessing they did indeed do some munching on your tasty roots. The eggs are usually laid in the late summer and they where growing this fall. Young juveniles I'm sure they had an appetite.
Luckily you re potted and found them. They could have caused more damage in the spring. Since you still have roots and the tree is acting well, i'm sure it will be fine.
I use a Mycorrhizal Fungi on the roots of newly potted trees. Reputed to help develop expansive root systems.Seems to help. Your tree being already established may not need it. I however do use it on a established tree if I am doing a lot of root work when repotting. Hope that helps John

I appreciate the response!

I'm concerned because there weren't any of the white, hair-like roots.

Do you use a particular product? I'll do a search on Mycorrhizal Fungi.

I'm going to have to do something about the other, long-term, potted maples!

I picked up a rhododendron in a trade that was infested with strawberry root weevils. My solution was to put the rootball in one of my ponds for 24 or so hours. That's long enough to drown the grubs, but not too long to drown the roots of the rhododendron. No pesticides needed. A large tub or bucket would work just as well.
Mike

Now that's a good idea!

A good soaking never hurt a JM. :)
Thanks.