WordPress Importer – User passwords not correct

I’ve run into an odd issue which I don’t understand.

I have exported a site from my multi-site installation (using wordpress’s standard exporter) and imported it into two new single site installations, one running 4.7.5 and one running 4.8.

In both instances existing users can not log into the new sites using the same password as the old site.

So I exported both the old and new user tables and the user_pass fields are different?

So as a test I copied the user pass from my test account on the old site to my test account on the new site and now I can login without issue?

So my question is, is this a know issue with the WordPress Exporter and Importer? Or is it something I’m doing wrong (I can’t image what that could be as everything else comes in fine?)