Microsoft To Do – Cannot join shared lists between two personal Microsoft accounts

Since both accounts are personal Microsoft accounts and you have tested sharing in both directions across multiple devices and browsers, this is likely an account classification or Microsoft To Do backend issue.

Recommended next steps:

Check both accounts at https://account.microsoft.com to confirm they are personal Microsoft accounts.

Verify that neither account is accidentally associated with a work or school organization.

Contact Microsoft Support and request escalation to the Microsoft To Do technical team.

Ask Microsoft to investigate the account identities and sharing permissions on the backend.

Suggested wording for support:

"Both accounts are personal Microsoft accounts, and the sharing issue is reproducible in both directions. All standard troubleshooting has been completed. Please investigate whether the Microsoft To Do sharing service incorrectly classifies either account as a work or school account. I request escalation to the product engineering team for backend investigation."

Important: A Microsoft 365 Family subscription does not automatically change a personal account into a business account.

I would avoid repeating cache-clearing or application-reinstallation steps, as your testing has already ruled those out.