Site Access "Request for" General vs site name

I'm logged in with my global admin account, which is also site admin for the given site, as well as an owner of the site. The admins group is set up to receive site access requests. My account has all the permissions it needs. The problem seems to either be the fact that it's a guest user requesting access (unlikely as the site has other guest users in it and is set up to be able to share with guest users) or the way in which they're accessing the site.

When I look at the access requests for this given site (a Team site), I see a guest user with the "Request for" column value "General" and I see a local user with "Request for" listed as the site name (Revenue Recognition External). In my screenshot the top account is a guest user and the bottom account is my user account. As site admin, when I click Approve for the guest user (with "Request for" = General), I get the error "Request approval failed" (screenshot attached). When I click Approve for the user account (with "Request for" = site name), approval is granted. 

These aren't the only accounts this has happened with; another guest user with Request for set to General also failed approval and other local users with Request for the site name approved without issue. 

It doesn't appear to be a guest user issue because I'm able to add them to the site / team and they can access it without a problem. It seems to be an issue with how they're initially accessing the site, perhaps? Why would "Request for" be set to "General" for them instead of the site name? Anyone run into anything like this?