The user version works perfectly. The group version correctly removes the existing tab and adds a new one, but the new tab leads to a 404 page when clicked. I was able to replicate on a fresh Docker install of WordPress 6.5.4 and BuddyPress 12.5.0 with no other plugins (besides the one I’ve made, which is one file) and the Twenty Twenty-Four theme. Thanks in advance for any help.
OK, I solved it, after way more effort than should be required. You have to set the subnav item’s screen_function to ‘groups_screen_group_admin’, then add a hook to ‘bp_screens’ that checks for your tab name and returns if not found, then hook your actual HTML to ‘bp_after_group_home_content’.