It is generally recommended to disable all other password managers (including those native to the browser) when using Bitwarden.
Incidentally, the same problem can be caused by Bitwarden’s browser extension if you save your Bitwarden master password in the Bitwarden vault, if you have enabled the “Autofill on page load” option, and if your URI match detection method for the vault.bitwarden.com URI is set to “Base Domain”.