Stonehenge, you're probably seeing tickets about this...
User tries to sign in to mailbox and receives error "OwaUserHasNoMailboxAndNoLicenseAssignedExcept ion"
https://portal.office.com/AdminPorta...lerts/EX280059 says "We've identified that two separate processes are attempting to update a user at the same time, causing contention when applying their licence."
https://portal.office.com/AdminPorta...sages/MC281145 is titled "Retirement of (Azure AD) Graph and license assignment operations and updates to license management APIs"
Gee, I wonder what the cause might be.
Thanks, Microsoft!