Is it just my confirmation bias, or do we disproportionality call female politicians sa pamamagitan ng their first names rather than surnames?

I think you may be onto something, actually. I personally noticed a lot of "Hillary vs Trump" back in 2016 and thought it was weird, but didn't immediately connect it to their genders. My initial guess was that people call her Hillary rather than Clinton in order to differentiate between her and Bill Clinton, who was already known sa pamamagitan ng his surname. But regardless, there does seem to be a theme going on.

I don't know if there's slightly less of this in the UK (particularly over the last three decades)? I can think of two British prime ministers who stand out as breaking this rule; Thatcher (often called sa pamamagitan ng her surname) and Boris (often called sa pamamagitan ng his first). We also have a lot of prime ministers who quite commonly seemed to be referred to sa pamamagitan ng their full names (one of which is Theresa May, but also most of the men who have served as prime minister in my lifetime such as John Major, Tony Blair, Gordon Brown and David Cameron).