For anyone wondering if this is a browser bug. Apparently it's not and it follows current W3C specs. What's strange is at first it was just inconsistent between the browsers (in some it worked as intended) and then all of them started to follow the counter intuitive W3C rules. There seems to be no clear explanation if this is actually intended logic, a side effect of some implementation problem or just a dumb omission.
Also position: fixed gets broken not only by transform but also by filter and perspective property put on any ancestor as explained here.
See: https://www.w3.org/TR/css-transforms-1/#propdef-transform and
https://www.w3.org/Bugs/Public/show_bug.cgi?id=16328 and https://github.com/w3c/csswg-drafts/issues/913 for more info on this issue.
