libGDXLibGDX doesn't support <objectgroup><objectgroup> inside <tile> yet<tile>, sorryyet. However, it could come, soon.
Please see Add handling of <objectgroup> in <tile> in in in the com.badlogic.gdx.maps.tiledcom.badlogic.gdx.maps.tiled package #3751 and Tiled map tiles objects support #4542 for details.