Disappointed but not surprised with S19. I’m aware that it’s basically a soap and having watched Greys I know what they’re like with sex but can’t lie it stings, especially with a wlw ship. I hate when shows use this plot in general it’s boring and obvious. Instead of this, it would have been so interesting to see Maya acting in or even just really go at it with Carina(not Jack like, wtf) and tie in proper Carina backstory and address Maya’s instinct of pushing people away when she’s pissed.

No, yeah, I get it completely. After a day to think on it, I think I can articulate why exactly I was so disappointed:

1. It was a freaking shock! It wasn’t foreshadowed at all, it didn’t even occur to me to worry about it, the way they’d written Carina and Maya and even Jack in the last couple of months didn’t at all imply there’d be this intersection. 
2. As you said, no matter how many times it’s reiterated how common a trope this is in this franchise, there are just historical connotations when it’s to ruin a wlw ship. Even if not intended in the same way to get rid of this relationship, I can guarantee that many of us were immediately shoved back to when that’s what would happen, f/f relationships deliberately sidelined and diminished after a token appearance.

   That Maya’s bi doesn’t change that. To quote my friend in a discussion we were having: “It’s tough, right, because bi characters obviously like both men and women and their choice to cheat can be indiscriminate–but it’s not as if like, that nullifies the gendered context beyond the show, in terms of media history and perception and tropes.” And in fact, it just swings into the cheating bisexual trope. Ironically, I was trying to offer counterexamples to my friend of straight Grey’s characters that cheated and brought up Teddy and then remembered they’d suddenly revealed she was bi too, so now both shows’ latest cheaters are bi.

3. They’re going somewhere so deep without any intention of really exploring it. This has always been one-sided and Maya’s story, which, again and again, we’ve acknowledged and accepted it, it’s her show, her arc, but they’ve now made this into a big thing for BOTH of them, such a deep betrayal of Carina, but we still won’t see Carina’s side of it. Like you said, instead of making this final conflict something core to them, involving Carina backstory, they did this instead.
4. Honestly, I resent them for taking this fun little ship and draining the life from it this quickly. In one move they’ve ensured the remaining fans approach it with wariness and take it off rec lists.