Unexpected value change event in DateField with MONTH resolution

The following code fires a value change event in Vaadin 8.9.0, while it didn't fire it in Vaadin 8.8.6.

DateField df = new DateField();
df.setResolution(DateResolution.MONTH);
df.setValue(LocalDate.now());
df.addValueChangeListener(ev->{
	System.out.println("OLD:"+ev.getOldValue()+" NEW:"+ev.getValue());
});

The question is whether this is a regression, or it is valid to fire an event in this scenario
(since the value is "rounded" to the first day of month, thus it's different from the value that has been set) or it isn't valid (because the resolution was known when the value was initially set)

Actual behavior: a value change event is fired
Expected behavior: ?

(Browser version: Chrome)

Reproduce the provided DateField example with DateResolution.MONTH on Vaadin 8.8.6 and 8.9.0, recording whether the listener fires and the old and new values. Then trace DateField's value and resolution handling to determine the intended behavior. Done means the expected event semantics are documented and, if a regression is confirmed, covered by a regression test.