What's weird to me is that Google itself has excellent IPv6 support. Every Google web site, API, or other service I've come across fully supports IPv6. Compute Engine VMs are the outlier.

They actually utilize this for their "private Google access" system, which allows you to access Google services from a compute engine VM without a public v4 address. The VM's private v4 address gets mapped into an IPv6 address, along with some extra bytes identifying the customer's network. (You can see this by setting up such a VM and accessing an AppSpot app that echos your source IP.)