Even now, in 2024, it is a relevant question. I however suspect that appengine somehow operates on a region level, not a zone level. While allocated instace might be in one zone or another, it is either random or unknowable. At least I failed to find in google documents any mention of a zone.

See this stackoverflow answer - it basically says the same. Are Google App Engine Instances Multi-AZ by default?

This means your best bet is to choose any zone of the correct region, and since it is the same data center, your timing will be ok. Not sure how it impacts your ingress pricing though.