Is it normal/expected for a resource import to require specifying attributes that weren’t in the original definition? I’m moving a resource between projects by copying over the code with an import option, and finding I have to add lots of attributes (mostly default values, some generated by the provider) to get to an importable zero-diff, even though those attributes weren’t in the original definition which shows zero diff after refresh.

Yes. It depends on the provider and how picky it is, but yes, it's pretty common to have to do this.