Depends how are you doing your Authentication ? (eg you might want to include the code your using)
It maybe something is different in terms of federation, conditional access policies https://learn.microsoft.com/en-us/azure/active-directory/conditional-access/overview for that customer or they may have reduced the refresh token lifespan https://learn.microsoft.com/en-us/azure/active-directory/develop/active-directory-configurable-token-lifetimes
The signin logs https://learn.microsoft.com/en-us/azure/active-directory/reports-monitoring/concept-all-sign-ins might help tell you more about what's happening during the authentication for them