Copyright of USSR-made works is not public ___domain; tracing the owner is tricky at times, but [obviously] works where the copyright inheritor is hard to trace are pretty much the opposite of public ___domain - you're not allowed to copy w/o permission, even if that permission is tricky to obtain.
The "made by gov't -> public ___domain" is an USA-specific detail that doesn't apply in most other places.
If specific people owned the copyright, then it's as usual for any other works; if a specific USSR institution owned the copyright, then the ownership will be determined by the early 1990ies transition/reorganization laws and contracts; if 'USSR' owned it, then it's owned by Russia afterwards, not public ___domain.
The "made by gov't -> public ___domain" is an USA-specific detail that doesn't apply in most other places.
If specific people owned the copyright, then it's as usual for any other works; if a specific USSR institution owned the copyright, then the ownership will be determined by the early 1990ies transition/reorganization laws and contracts; if 'USSR' owned it, then it's owned by Russia afterwards, not public ___domain.