We recently updatet Bitbucket to 7.16.0 and since the update Edge and Chrome request a User Certificate to login. If we klick "Abort" the login Mask is appearing. But its a Bit annoying.
Either this is something implemented on your load balancer or reverse proxy server, or something that a third-party app you installed on Bitbucket Server provides - it's not something that Bitbucket Server would do out of the box.
Premier Support Engineer