My GitHub app has the member webhooks enabled but no delivery shows up in the log under advanced settings when the invitation was accepted which according to https://developer.github.com/v3/activity/events/types/#memberevent should happen.
The account the GitHub app was installed on was a personal account - not sure if this is the reason?
It's a little confusing, but there's a separate event for this:
Let me know if this resolves your issue!
Actually, I think I'm wrong since you're referring to Outside Collaborators. I'm not sure why that's happening for you, but you could test it on an org installation and see if it's due to the repo being on a personal account.