Cdc Likely after the installation of Thunderbird for Gmail (Well, I think).
Thunderbird's "sign in with Google" feature opens a custom tab with accounts.google.com in your default web browser in order to connect to your Google account. Custom tabs share state with the default web browser. Since you signed in to google.com in a custom, non-incognito tab, you signed in to google.com in your default web browser. If you enable "open external links in incognito" in Vanadium, and Vanadium is your default web browser, custom tabs will open incognito, but you likely didn't have that option enabled.
On custom tabs: https://developer.android.com/develop/ui/views/layout/webapps/overview-of-android-custom-tabs
Cdc Still a bit concerning that it has signed into Vanadium itself somehow without Google Play.
Google Play is not required to sign in to a Google account on google.com.