gpg(1) says:
show-only-fpr-mbox For each valid user-id which also has a valid mail ad‐ dress print only the fingerprint and the mail address.
however, it also shows invalid user IDs:
0 dkg@alice:~$ gpg --list-keys 7B96D396E6471601754BE4DB53B620D01CE0C630 pub rsa1024 2006-01-01 [SC] [expired: 2011-06-30] 7B96D396E6471601754BE4DB53B620D01CE0C630 uid [ expired] Werner Koch (dist sig) <dd9jn@gnu.org> 0 dkg@alice:~$ gpg --list-options show-only-fpr-mbox --list-keys 7B96D396E6471601754BE4DB53B620D01CE0C630 7B96D396E6471601754BE4DB53B620D01CE0C630 dd9jn@gnu.org 0 dkg@alice:~$
I would expect from the documentation that the lines emitted would only be e-mail address/fingerprint pairings of fully-valid user IDs (i could imagine it doing something with user IDs with marginal validity as well, but unknown or known-invalid user IDs seem well beyond what the documentation offers).
Either the code needs to change, or the documentation needs to change to make it clearer what the intent of this feature is.