I can reproduce this consistently now: Save auth token to keychain... Kill app entirely & put device to sleep... Wait at least 20 minutes... Restart the app & the auth token has been wiped... I'm on an iPad (6th Gen), iPadOS 15 beta.

Perhaps the documentation should be updated. https://developer.apple.com/documentation/scenekit/scntechnique#1965885. One of the main examples given is deferred shading to achieve SSAO which is exactly what @pprovins is trying to achieve.