Shared Calendars No Longer Visible

For years, I've shared Outlook calendars between my Outlook accounts and with my husband's, without any issues. However, since mid-June the shared calendars have disappeared for no apparent reason.

The sharing permissions are still in place, but the calendars are no longer visible or available to add on Android, in the Outlook desktop app, or on Outlook on the web.

I've spent hours Googling and tried all the usual troubleshooting steps, but nothing has worked! As a workaround, I now have to invite all @outlook.com accounts to calendar events just so everyone can see them and avoid double bookings.

Has anyone come across this before and found a fix that isn't one of the standard "how-to" suggestions?

Note: these are personal paid Microsoft accounts.