Your system, Windows 8.1, when detect than is needed login to use internet, automatically try to open "http://go.microsoft.com/fwlink/?LinkID= ... lcid=0x409" (Internet Explorer Home Page) without ask anything, and obviously wants you to see Bing (the Microsoft search engine) caring about the selected page as your homepage inside your default browser.
If you use chrome, firefox, opera or others, Windows launch the "default program for open urls" (not the default browser... on new Microsoft language...) and pass as parameter http://go.microsoft.com/fwlink/?LinkID= ... lcid=0x409
You try to solve this: change default web pages of Internet Explorer with "https://www.google.com".
If you not solve, ask Microsoft, not this forum.
MikroTik HotSpot simply after login redirect you to the page you try to open without login.
The problem you have is not due to RouterOS on any way.
I'm Italian, not English. Sorry for my imperfect grammar.