Upon experimentation, I have observed that there is a second factor contributing to the issue. After autofilling, the Password field (EXP_password) is given focus using some mechanism that the DOM event listeners are interpreting as a mouse click into the password field. This triggers the onclick listener to execute the command that clears the input field.
It is unclear to me whether this “virtual click” into the password field at the end of the auto-filling is caused by the website’s HTML/Javascript code, or by Bitwarden’s auto-fill code. The virtual click into the password field happens only if the vault’s login item specifies a value for the password field (whether through the regular Password field, or through the custom field EXP_password). However, it doesn’t matter in what order the fields are filled (I tried removing the regular Username and Password fields, and then adding/re-arranging custom fields for EXP_MEMBRE_MODE, EXP_email, and txt_recherche) — it is always the EXP_password filed that receives focus upon completion of the autofill action. And this clears the field.
I wonder if Bitwarden is deliberately creating a virtual mouseclick into the password field after autofilling a password (perhaps to facilitate manual addition of a password pepper)? If so, it may be a fixable problem: Bitwarden could give the password field focus without virtually clicking into it, and then send an End-of-Line code to place the cursor at the end of the autofilled password.
I have submitted an Autofill Failure Report about this issue on Github. Maybe that will eventually lead to a solution. But as @Peter_H has suggested, you may also want to contact the website owners to file a complaint that the design of their website prevents use of password managers (and therefore contributes to making the internet less secure). Contact form:
I still have no answer from the forum-juridique contact. To temporary solve the issue, I have disabled javascript for this website and I can log on with Bitwarden.