What’s with https://www.copyright.gov/title17/92chap1.html#104a ?
So, a restoration may have a different copyright status than the orlriginal work, if I understand that correctly.
Also the copyright law seems to have some hints that copyrights in other countries may also apply in US territories. So, it may also be possible that the restoration also work is protected die to the chain: protection in original country => protection in the US…