Can’t share Reminders list with my child

Previously shared Reminders lists no longer appear on my son’s iPhone, Mac or on iCloud.com since I don’t know when. They do appear on my daughter’s devices. All devices are running the latest OS.

I deleted him from the sharing list and tried to reinvite him. Unlike with my daughter, he didn’t receive a notification, only an email. But when I click the link, he’s prompted to log in to iCloud.com (which we can’t do on the iPhone…switching to the Mac), but we get an error that says “This reminder is unavailable. The owner stopped sharing, or you don’t have permission to open this reminder”.

I’ve logged him out and back in to his iCloud account on all devices, and tried deleting the app and reinstalling on his iPhone and mine (advice I found online), and also turned Reminders off and on in iCloud settings.

I also tried deleting my daughter from the shared list and reinviting, and it works fine. It appears as though all devices are running the same version of the app, but I did notice the “managed shared list” is very different on my son’s phone. I think all reminders lists are “upgraded”, but I am running some older Macs so I’m not sure if that’s a factor.

I’m out of ideas. Anyone know what to do here?