bitwarden fills the password into both the unsername and password boxes on a financial website. I have even deleted the username in bw and reentered. All looks good in bw but won’t fill correctly. Found same complaint from 2019 that thought problem with HTML field height size but it was beyond me technically and apparently no solution.
Website developers are able to use a whole variety of names for fields. Password mangers do a good job of detecting them, but cannot be pre-programmed for all of them. Sometimes they need to be fine-tuned. This generally involves re-naming the username and/or password fields matched in Bitwarden for the particular site, after which Bitwarden should work fine.
If you do not think that you can do this fine-tuning then I think the only thing you can do is find someone who can. Alternatively, if you post the URL of the website then someone might be kind enough to take a look at it and see if there are any likely fixes.