No, it’s not a bug. GtkButton already has an event controller that is used to handle pressed/release, and will take over the event sequences. Adding another event controller is not going to work. That’s why @pyzahl added an icon inside the button, and used an event controller on it.
If you’re taking over what GtkButton is doing, then I strongly recommend you write your own button-like widget—just like I suggested you last week.