I have been assuming that something is wrong with Bitwarden and Chrome-based browsers, particularly Vanadium, but after testing some things out I have deduced that Chrome browsers on Android don't seem to play well with autofill.
I was very excited to read that Bitwarden had added support for Vanadium. On one of my devices with GoS I was running Vanadium (org.chromium.chrome) but inline-autofill was not working. I decided to take a second device and do a fresh install of GoS so that it would have a package name of app.vanadium.browser, which this github issue tracker specifically states support for.
I've tested various Android browsers' autofill ability using Bitwarden and KeePassDX.
Within GoS at Settings->Passwords & accounts->Autofill service I alternately selected Bitwarden or KeePassDX form autofilling, depending on which password manager I was testing. Inside the Bitwarden app at Settings->Auto-fill I have Autofill service and Use inline autofill enabled. Regarding inline autofill, I am using Gboard with network permissions revoked. Similarly, inside the KeePassDX app at Settings->Form filling->Autofill settings I've enabled Inline suggestions.
With these settings, neither Bitwarden nor KeePassDX inline-autofill work for:
Bitwarden/KeePassDX inline autofill does, however, work with com.vivaldi.browser, though there are apparently some modifications to its Chrome base.
All Firefox-based browsers that I tested (Tor Browser, Firefox, Mull) work splendidly for autofill using both Bitwarden and KeePassDX.
Many apps on my phone do not work for autofill and I can only assume that these apps are Chromium-based, although that's just a guess.
This issue is a huge problem IMO and I'm wondering if anyone out there has any insight into what the outstanding issue is. Is this an AOSP thing? Chromium-base thing? GoS thing? Bitwarden/KeePassDX thing?