I am not able to replicate the problem you're reporting. Make sure your CustomEntityProperty and CustomMetaDataProvider classes are properly configured.
I've attached a demo application as guidance for you, it's already configured to deploy to UWP (be sure to do a rebuild to restore NuGet packages).
If after reviewing the code and making necessary changes to your application doesn't fix the issue, please update the demo so that it replicates your problem and sent it back to us for further investigation.
Lance | Tech Support Engineer, Sr.
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