Found the solution:
https://bitwarden.com/help/article/install-safari-app-extension/
Pay attention to the newly added note regarding the 2021-03-11 release. Bitwarden really should have added this warning to the to upgrade notification. The message needs to be far more obvious. Many Mac customers are going to be caught out by this.
The Safari extension is no longer available via the .dmg installer. You must install via the Apple Mac AppStore.