Vaultwarden is NOT Bitwarden (and also not a fork of it). It’s a separate, independent, program that behaves like the Bitwarden server, so that you can use e.g. the Bitwarden clients, etc.
So even if Bitwarden (server) is updated, this doesn’t mean that Vaultwarden has to update anything. They don’t have the same bugs or security fixes, etc.
Also, the version visible on the web interface is the web-vault version and not the Vaultwarden version it self. Besides that, if you want the latest version i suggest to use the testing tagged images which are always build based upon the main branch.