I've seen it done several times now, once well and a couple times not-well.
It's when you have a two-part story, and you spend the first part getting your main couple together. And then just when they're happy, you do something awful to split them up again, and they spend the second half of the story trying to get back again, only this time they're usually misunderstanding and hurting each other along the way, instead of finding ways to overcome external odds.
Gonna be discussing Gearbreakers/Godslayers, Witch from Mercury, and Malice/Misrule. Spoiler cut.
So in Gearbreakers/Godslayers, the breakup wasn't too bad, because it was very clear that brainwashing and mind control was a huge part of it. So it didn't really feel like the characters were actually hurting each other, and they still got an epic reconciliation.
I've previously covered that I was pretty bummed by the second arc of Witch from Mercury, because I felt like the Suletta/Miorine relationship, which was the heart and soul of the first arc, took a back seat to the Quiet Zero/Prospera/Eri plot. It never hit the emotional high of their chase from episode 11 again, and I think it needed that. Even if they stuck to Gundam tradition and didn't let them kiss, another big confession to fix Miorine breaking Suletta's heart was definitely warranted.
I really, really enjoyed the first book of the Malice & Misrule duology. It was a perfect fresh retelling of classic fairytales, combining Cinderella and Beauty & the Beast to have a misunderstood social outcast fall in love with a princess and save her from her curse. But then at the end, it ripped all their victories out from under them to set up the second book. Which honestly felt like an unnecessary sequence of them misunderstanding and being cruel to each other, with borderline cartoonish side characters who didn't mesh with the feel of the first book, just to have drama for a second book, with a bittersweet ending.
And yeah, maybe I'm just feeling hyper-conscious of it because I finished G Witch and Misrule at about the same time, but I don't like this trope. Let them just have their happy ending, don't fuck them out of it to create a second story unless you're somehow going to make it even happier.