Calendar doesn't remember which Office 365 account I selected as the default

I have an Office 365 personal account and and Office 365 work account. Every time Apple decides to update the Calendar app, it’s a crapshoot which Office 365 account Apple will pick as the default account again.

Apple needs to fix the bug in their app update process so that Calendar remembers which Office 365 account I selected as the default. I don’t want Work knowing about my Personal appointments!

[Re-Titled by Moderator]