For anyone who wants to build “sandbox-safe” AUs, the .plist file also needs to be updated with the sandboxSafe flag added in the right place.
I’ve made a patch exposing this as an option in the Projucer in the SR branch: https://github.com/soundradix/JUCE/commit/97e2a2acb93e8f38c0df8f3b17e93f04c7b79aac
Cheers, Yair