In the US Kei cars/trucks are handled differently by each state. We had to vote to get them approved to drive into the cities here once enough farmers had them and kept begging for the laws to change. Although for us it stems from having them imported as farm equipment.

In 2021 Maine cancelled all of the registrations for these vehicles, so it's a local regulation. Sounds like it was a wording issues. Looks like there is a bill introduced to fix this: https://gearjunkie.com/news/maine-bill-reinstate-revoked-del...

Registering it does depend on the state: I've heard some states will prevent you from registering them due for emissions or safety reasons, even though they're federally legal to import