Post-film, Grindelwald escapes and regains Credence's trust. They're not together, but like in the film, there's lots of standing extremely close and speaking in hushed voices, coupled with petting. Credence is in love with Grindelwald, who knows this, and uses it to his advantage without actually taking their relationship to the next level.
In the course of their travels, Grindelwald occasionally comes across someone he wants to manipulate, and ends up acting the same way he does towards Credence towards the person he wants to manipulate. Credence doesn't like this, but since he and Grindelwald are not actually together, keeps silence. One day, though, the jealousy gets to be too much and Credence loses control, killing the person Grindelwald's coming onto. When Grindelwald confronts him, Credence gives the other wizard an ear full about how he knows Grindelwald is manipulating him and how much it hurts to see him act the same way towards other people.
Since Credence is so powerful, Grindelwald can't risk losing him, and ends up agreeing to stop flirting with other people.