Not an idea, just and issue. I have a published report with a single select filter that works fine when you access the report directly. When I add the report to an app the app version of the report reverts the filter back to a multi-select filter. I have confirmed that the both the "Show 'Select all'" and "Multi-select with Control" are turned off before changing the filter to single select. Any ideas on how to fix this?
With my tests, I can't reproduce your issue. You can try to delete the report in the application, republish the PBIX to the workspace in Desktop, and republish the report to the application. If the problem persists, please provide your detailed steps.