The api doc/signature, from Styleable:

    /**
     * Return the pseudo-class state of this Styleable. CSS assumes this set is read-only.
     * @return the pseudo-class state
     */
    ObservableSet<PseudoClass> getPseudoClassStates();

Note that it's observable, so client code can assume that it can install a listener simply like:

// instance fields
SetChangeListener myListener = ...
Node node = ...
private void someInit() {
     node.getPseudoClassStates().addListener(myListener);
     ...
}

Which fails: at some time after init, the notification stops. Reason is that Node returns an ummodifiable wrapper around the states (which is fine) that's recreated on-the-fly on each call (which is a bug, because the returned set has no strong reference and can be garbage collected, see https://stackoverflow.com/q/53580773/203657)

Here's a simple test that fails:

    @Test
    public void testPseudoStateGarbageCollection() {
        // any node will do ..
        TableView table = new TableView();
        assertSame("must return the same instance on every call",
                table.getPseudoClassStates(), table.getPseudoClassStates());
    }

A workaround is to keep a strong reference to the set in client code. Ideally, that should happen in Node (just as TableView.getVisibleLeafColumns does) - subclasses can't because the method is final. At the very least the unexpected behavior must be strongly documented.