I have 2 Users in the Site Owners Group for a Site with Full Control permissions. Most of the 'expected' controls are working, however when the Site Owner tries to view 'Site Permissions' they receive 'Access Denied' Message. This happened after adding Community site features to the Site through the 'Manage Site Features' Setting.
The solution that worked for me was restoring site owner access to the hidden Access Requests list. I do not know how permissions were lost in the first place. While on the Access requests and invitations page I opened Developer Tool to obtain the list GUID. I then entered the list GUID into the address bar and re-granted access to the site's Owners group. Step-by-step instructions with screenshots are available at https://jennyssharepointtips.wordpress.com/2017/11/08/restore-lost-permissions-to-access-requests-and-invitations-and-or-site-permissions/.