Certainly frustrating that Google enforces these sort of consumer protection feature on fully managed enterprise owned devices. In this use case we should be able to bypass these sort of restrictions since they're enterprise owned and locked down assets. Unfortunately thats not the reality however and we're stuck dealing with these kind of issues.

The first thing I would maybe try is to see if you can disable Google Play Protect on the devices and to see if that would then prevent this message from displaying. One issue with that approach however is if that does work I think disabling GPP is a manual process that can't be pushed from an MDM. 

If the GPP disablement doesn't work, you could maybe try and pushing the APK directly to the devices as a Package based install instead of using Managed Play. This assumes you have access to the APK.  If you are dependent on Managed Configurations you can push those as part of an Enterprise app Policy in newer versions of MobiControl without having to route the app install through Managed Play. 

Hi

We also encountered this issue. Ultimately, the manufacturer of the app had to submit a request to Google Play Protect to have the app classified as secure. (Play Protect Appeals Submission)