I had a SharePoint integrated SSRS instance. Lately some problems happened where the data source is losing the stored credential every night. It happened both to the shared data source and the custom data source.
When I manually reentered the credential, it would be wiped again overnight.
Turned out, the problem is because it didn’t use a domain account as the service account. Based on certain security recommendation, I configured the reporting service to use one of the built-in accounts. It turned out to cause the issue.
Solution: Use a domain user account as the reporting service’s service account.