I got some information from the related team.
The VSTS Content pack is based on VSTS REST API’s. Calls to those API’s were causing accounts to be throttled and introducing a lot of load on the service so we introduced a delay into the calls. Introducing the delay has caused some accounts to hit the timeout for data refresh in Power BI.com which is why the customers data set went from working properly to failing unexpectedly.
We are working on a new OData endpoint tied to the new Analytics Marketplace extension (https://marketplace.visualstudio.com/items?itemName=ms.vss-analytics). The OData feed is currently based on WIT but will grow to support all data types. The OData endpoint is designed to address many of the problems with the content pack today but unfortunately the preview of the WIT OData endpoint is closed.
If you send me the customers VSTS Account URL, I can send it to related team and they will add it to the preview as soon as it becomes available.
Best Regards,
Herbert