On that website, the webpage has been designed so that the HTML code for rendering the login form elements literally does not exist when the page is loaded, but the code is injected into the webpage when you click the “login” button. Clicking the login button does not load a new webpage, it just changes the appearance of the current webpage. Thus, there is nothing for Bitwarden to auto-fill “on page load”.
I’m gonna put on my customer hat and say: i dont care about the specifics such as “on page load” and other technicalities.
Google Chrome (Vivaldi in my case) does autofill right after i press the Log In button whereas BW doesnt, so it’s a bug in BW.
Well, bugs should be reported on GitHub, but I suspect that you may be informed this is not a bug, because autofill on page load is behaving as designed. However, there is also a generic “feature enhancement” thread on Github for the autofill feature (Issue #1621), where you can access a form for reporting troublesome websites.
Alternatively, you can try to make this the topic of a Feature Request.
And if you have some ideas for how to implement a solution to this problem, you can always propose a Pull Request.