If y'all watched Jesy's documentary, you'll see that her relationship with the girls wasn't always positive. When they were in the studio recording for Bounce Back, Jesy kept beating herself down because she thought her verse wasn't good enough even though the girls and StarGate said that it was; she ended up recording several takes before she was happy with her performance. Jade (or Perrie), if I recall correctly, said that Jesy's not easy to work with because she's always worried about not being good enough and doesn't have the same expectations as them when it comes to their work.
Their working relationship, understandably, would become strained over time because they're four individuals with different personalities. It wouldn't have been easy for the girls to have always compromised for Jesy's perfectionism and image issues. Likewise, it was unfair for Jesy to guilt-trip herself for being miserable around three girls who were probably living their best life during promotional cycles. My view of their relationship is that they were simply co-workers who became great friends, but as time went on, it also became toxic.
It's understandable that Jesy would want to distance herself from the girls because she'd be triggered by what happened and what could've been had she stayed. As a fan, I'm sad about it because I've seen their relationship evolve over the years, but I'm also hopeful that there's a reconciliation down the line.