I've been having this same issue (see my post above). However I noticed on the August 2020 On-Premises Data Gateway notes state that the Mashup Engine has been updated and now matches the August 2020 version of Power BI Desktop. I went ahead and updated the Gateway to the new August 2020 version. I then tested a couple of reports that have been failing and now they are sucessfully working.
You can view the MS Blog post about the gateway update at https://powerbi.microsoft.com/en-us/blog/on-premises-data-gateway-august-2020-update-is-now-availabl...
Hopefully this resolves anyone who is having this issue.
