Identity Governance - Guest Access Review - No access to review

Question

Looking to setup access reviews - firstly for guests - I'm facing a peculiar response from about 25% of the groups in my initial test.

Not wanting to go all in and choose all, I hand selected about 40 groups containing guests, to be subject to review from owners of the groups. As mentioned, most of these were sent as expected, but some of the groups - again, doublechecked to be containing guests - just are shown in 'Identity Governance'->'Access reviews' with a warning that states 'No access to review'.

I'm pulling blanks as to what the 'problem' with the guest in these groups, that make them illegible for the review to be sent out.

Have anyone experienced similar or have an idea as to what I should look for in my troubleshooting? I can't really go forward with broadening the scope of groups, before I'm 100% that all groups containing guests will be sent to be reviewed by their owners.