Why would me ex whos in a new happy relationship still follow my social media & interact with me? & why would he stop once he knows I’m with someone?

If he’s happy in a new relationship but still contacts me and sees nothing wrong with it, then why suddenly stop when he finds out I’m also now with someone else too? If it was just friendly then what’s the issue?

Updates
6 mo
Any helpful thoughts guys? I’d be keen to hear from more mature and wider members with experience. Thanks 🙂
Updates
6 mo
Bump
Why would me ex whos in a new happy relationship still follow my social media & interact with me? & why would he stop once he knows I’m with someone?
Post Opinion