No error message, he can open the app but data not showing up.
Here is the situation:
SP source list is on siteone.
Powerapp created on sitetwo.
All the users are using the app with account connection @siteone
The person who can't see the data has not a @siteone.com account, so he is using @sitetwo.com account.
He has been given access/permission to the SP list on siteone using his @sitetwo.com account(and he can use the list with no issue), but when he try to use the app to manage the data, the data are not loading.