Can you see it in auval? Also, it’s super important that your AUv3 wrapper app has the Sandbox capability ticked. The projucer should do this automatically for you. But you will probably need to have a Mac Developer account and specify your developer team id in the projucer. If the wrapper app is not properly sandboxed then the AUv3 will not register.
I’ve seen the kAudioComponentInstantiation_LoadInProcess flag - but I could only get it to work if the AUv3 appex bundle was inside the host apps bundle and the AUv3 had a child bundle id of the host app (for example: com.roli.auv3host.auv3filter and com.roli.auv3host - the projucer automatically does this latter for the wrapper app and the appex bundle ids, for example). This limits the loading of AUv3 plug-ins to non third-party plug-ins only - which is not very useful. However, the Apple documentation does not mention any of this so it could be a JUCE bug.