I used to see GrapheneOS listed in Apps and About Phone (Pixel 4a) but no more.
Vanadium is still the default browser but I can't find any reference to GrapheneOS anywhere.
Is it possible that they slipped-in a change back to Android as an "update" that I thought was from GrapheneOS?

No. Updates are signed, you can't load an update from another operating system, so unless you actively flashed another operating system, and assuming you flashed GrapheneOS in the first place, there's no way you're not running GrapheneOS.

You can use Auditor to find that out:

https://attestation.app