I didn’t find much with my Google-Fu, but I found another issue related to outlook.com accounts and put two and two together to fix my issue.
If you run into a problem with a Microsoft 365 Exchange Online account trying to access shared calendars in OWA, but the calendars appear and then disappear – as well as you can’t see the Shared calendars option under OWA (outlook.office.com), Settings, Calendar (there should be Shared calendars here) and OR if you do have Shared calendars as an option, but when you click on it it says unable to load these settings – check the Accounts section under the OWA, Settings, Calendar section. There might be a Microsoft account, which will be a non-work/school account, aka outlook.com account, or an older email address that is probably the same as your work/school account. Remove it from the Account section.
Second, I highly recommend changing the Microsoft account’s sign-in username from [email protected], which is probably your Microsoft 365 work/school username, to [email protected]. You can do this via live.com by logging in to your Microsoft account (non-work/school account) and updating the settings there.