The debugging experience is much richer in the Flow portal (vs PowerApps portal). You look for the same flow run at https://flow.microsoft.com/ and you can click through to see which step failed and why.
I have created a basic Flow attached to a PowerApp. The Flow only has a "When a control is selected", which is attached to a button in the app, and a Get My Profile action. When testing the app, using my credentials in the Office 365 Users connection, I get one successful attempt. Then every attempt subsequent fails with an error message "The authentication credentials are not valid".
The following is the error message in the Flow:
The error copied from the Flow looks like a known issue we're fixing (basically, flows triggered from a PowerApp must be created from the PowerApps website). If you create a new manually triggered Flow from the PowerApps website, this error should go away.