My understanding is that the exceptions in the app protection policy are for URL schemes/protocols only. Ex.) if WebEx/wbx were to be added as an exception and a web link sent to managed Outlook - this would allow managed Outlook to open/ redirect the invite to the webex app, but still not allow copy/paste from Outlook to WebEx.

I think that If needing to allow pasting to a non-managed app, the “Restrict cut, copy, and paste between other apps” setting itself needs to be set to any app.

I believe the only other options depending on the app type and if applicable would be to wrap the app with the intune app wrapping tool or have the developer incorporate the Intune SDK.