Unable to use webhooks with geofence entry/exit

I'm currently trying to create webhook triggers based on entries and exits into and out of genfences, but whenever I get to that field the entry/exit triggers are grayed out and I can't choose them.

I'm able to use triggers for inside/outside geofences with webhooks, but I can't do it for entry/exit.

Is this a feature or a bug? Anyone else had success in finding a solution?