Bitwarden's "Unlock with Biometrics" (TouchID) appears to have intermittently triggered Secure Input Mode after enabling the feature in Firefox, Chrome, and Safari. It didn't trigger immediately, but rather after locking and unlocking my machine from what I recall.
Disabling "Unlock with Biometrics" across all browsers and the native app seems to have resolved the issue.
I'm using Bitwarden Version 2023.3.2 (8075) on macOS Ventura 13.6.6
So, thanks Peter. I used these commands and @Onan's Is Secure Input Enabled macro to find the unhelpful loginwindow as the culprit and knew it was time for trial & error. Thanks to you as well Onan.
Would leave a decision on whether to update the Wiki up to others.