People can be so fake. I feel your pain. These sort of things have made me, personally, distrusting of "friends"

Maybe the person is more upset by you leaving than you think, and maybe instead of ignoring they are just super upset and handling their grief differently.

Know the difference between friends and colleagues from work. A friend is someone who wishes the best for you regardless of where you work. A colleague is a work acquaintance. Once you “switch teams” and work elsewhere your work colleagues are not going to automatically be your friends. In the working world we need both friends and work colleagues. You will normally have more colleagues than friends, but those will change as you change employers.

This is why you NEVER shit where you eat. A co-worker is just that, and should be treated as such.