Actually, there's another issue:

class Test {
    public int $prop { set => $value; }
}

$test = new Test();
var_dump($test);
foreach ($test as $key => $value) {
    var_dump($key, $value);
}

This currently prints NULL, but the property should be skipped instead... I'll have a look tomorrow.

Ok, both issues should be fixed now. I changed the implementation to set data/key to null on reset/forward, and fetch the data (if it wasn't already) on valid/get_current_data. Fetching the data includes moving forward further if the data is unavailable, both for declared and dynamic properties.