SharePoint source forgets credentials on scheduled refresh

I have a report that connects to a combination of SQL Server data sources, and a SharePoint Online list.

I ma finding that the data source for the SharePoint Online list is constantly forgetting its credentials, causing the scheduled refresh to fail.

In Power BI Service, on the report settings, under gateay connection it says "Gateway or datasources not reachable". For the 3 sources that I have (2x On-Prem SQL Server, 1x SharePoint list" - I can see each are successfully mapped to a Data Source in my On-Premises gateway. When I go to manage gateways & go to the data source settings for the SharePoint source I'm getting the details "invalid connection credentials". When I select edit credentials - I select OAuth2 as the connection method, select my account and it proceeds without any further prompts, but the error doesn't go away.

From here I open my Power BI Desktop file, attempt to refresh the data and I'm prompted to sign in for my SharePoint source - it says "the credentials provided are invalid" - under the Microsoft accouint tab, I select sign in as different user, select my account, select the appropriate site collection as the level this applies to and select connect (point to note I use the same account every time). This works ok and the data refreshes (and I can see the updated refresh time for the report in Power BI Service).

So it works manually, but not automated. It needs to update every hour (and I can't do that manually). Can anyone help as to why it won't update on the scheduled refresh??

I can also confirm all privacy level settings, for all data sources, in the service and under options in Power Bi Desktop are set to Organizational.