I think this scene took place between her being outed as a virgin (7x03) and Jackson punching Alex (7x08). Jackson was one of the ones laughing at her for being a virgin, so they can't have been too close at that point.
I agree it wasn't enough, but there was a bridge between them the fooling around and April getting served- Jackson and April arguing by the car after a therapy session. Jackson calls out April for never taking responsibility or apologizing. She continues to be pissed off. It seems that he's had his fill- he knows she's never going to get it, so he's out.
As a Japril fan, I was happy to have the screentime, and SD and JW were so enthusiastic about the episode that it seemed kind of ungrateful to complain- but honestly, 12x11 was a bit of a mess. Some of those flashbacks got April totally wrong, they were writing her from the perspective of Season 12 April, as she is now, overly defensive and insensitive, rather than how she was portrayed from Seasons 6-10.
April's reasoning for leaving/coming back/etc has never made sense. She was only shown in a handful of scenes after the baby death episode, mostly seeming to be doing OK, then there was a shot of her looking at babies in the hospital nursery and she needed to go off to war for a year. Then she comes back, decides to leave again (it's her "calling"), but then, offscreen, somehow she decides she needs Jackson (or rather, her marriage) so much she's willing to become practically a stalker? Terrible writing.
12x11 would have been a wonderful opportunity to retcon some sense into this storyline, but they missed it.