Cannot change data source credentials to anything different than current credentials because of SSO

Update after spending 2-3 hours with Microsoft support this week. I am grateful for their help and patience. I believe they gave me a workaround but that this issue that remains unresolved.

Changes made:

- The service account had been locked out and was re-enabled.

- My own account did not have access to the Sharepoint data source and was granted access.

- The setting "Allow user's cloud data sources to refresh through this gateway cluster" was enabled on the gateway cluster.

After those steps, support had me deploy a copy of the report (while logged in to Power BI service as the service account) and the report was successful.

I also attempted to change credentials on the initial report deployed by me, logged in as me, and as reported throughout this thread, I was not prompted for a password. The report refreshes successfully. (as would be expected if my own credentials were still being used.)

Because I cannot see which credentials are actually being used on the data source, I cannot report success in the objective mentioned in post 5: "Publish a report that uses a generic O365 account (different than the one used to publish/manage the report) to query a sharepoint data source."

At this point I do not want to experiment further with changing credentials in order to prove this issue exists (at least under some scenario) in case I have to spend more time fixing this issue.