The first and last tanda “rule” is just a societal norm in partner dancing.
I think it’s fair to speak up if it bothers you that your girlfriend doesn’t save you these dances. Perhaps she grew up in a time when there was no partner dancing.
This is an important point. It isn't just about when ("a time") but also where (a place), and the social context of that time and place.
I'm in my mid 50s. Where I grew up and formed my social identity through the late 1970s and the 1980s – a smallish and somewhat rough Canadian port city whose main industries were, at the time, tourism and the navy – social dance didn't exist. Well, it did, but among people my teenage self saw as
fantastically old (in their late 70s and onward), largely in retirement homes. My parents didn't do it; my grandparents didn't, either, because it was something "people in those homes" did.
On the other hand, I went out a lot. I danced with big mobs of punk rockers and new wave freaks for years. Later, I hung out in the university nightclub scene some. In those contexts, dancing was a melee with no clear partner involved, and not closely related to anything to do with couples. Really, it was a thing to do in gangs whilst drinking too much.
So, I grew up with no sense of the norms for social dance . . . everything I know about that, I've learned since commencing tango in 2013. The "first and last tanda rule" isn't one of those things.
In my couple, the major rule is "save the valses." I can dance my first tanda with some random woman while my partner checks out the shoe boutique. Because my partner has chronic illnesses that mean she gets tired and decides to sit and chat with friends long before she wants to leave, I usually dance my last tanda with some random woman, too. That lack of energy also means I dance most milonga tandas with other people. But
valses are strictly between her and me, and dancing a vals with someone else can be a problem.
Again that’s just the societal norm.
In a specific context, of course. Not globally, in every little city and town.