HTTP Auth: How is this supposed to work?

I’d like to use HTTP auth as a secondary security layer for a web app I run on my local network.

I have HAProxy running on my pfSense router. I configured HTTP Auth such that it prompts for the username and password to access the specific domain, but when I enter the credentials, it simply reloads and asks me for them. ad infinitum.

When I turn the HTTP Auth off, it loads the web app as expected.

What am I doing wrong/misunderstanding about this?