It seems to also trigger the "window.onunhandledrejection" event. I suppose they're based on the same hooks.

https://jsfiddle.net/c22svqh2/4/

And the "window.onrejectionhandled" is never triggered either:
https://jsfiddle.net/c22svqh2/6/

That's not a bug.

Consider this:

window.onunhandledrejection = function(error) {
  console.log('Unhandled:', error.promise);
}

var promise = new Promise(function(resolve, reject) {
  reject(new Error('Rejected'));
});

promise.then(function() {
  console.log("Success !?");
});

promise.catch(function() {
  console.log("caught.")
});


The console output would be:

caught.
uncaught
undefined:1 Uncaught (in promise) undefined

The reason is that Promise.prototype.then also creates a promise. If it does not define a reject handler, it attaches a default handler. But that counts as unhandled, since the rejected promise can still be passed on.

Now consider:

window.onunhandledrejection = function(error) {
  console.log('Unhandled:', error.promise);
}

var promise = new Promise(function(resolve, reject) {
  reject(new Error('Rejected'));
});

promise.then(function() {
  console.log("Success !?");
}).catch(function() {
  console.log("caught after then");
});

promise.catch(function() {
  console.log("caught.")
});


Console output is now:

caught.
caught after then

You're right, I was thinking that promises returned from .then shouldn't get reported if they don't get assigned to anything, but most of the time that's actually a programming error.
It's safer to chain a .catch at the end anyway.