The Overgrowth (Dummy) ability is based off Entangling Roots. Therefore, it has the same casting command. So when the dummy is ordered to cast Entangling Roots, it will cast the Overgrowth (Dummy) ability, because to the trigger, it's the same spell.
The unit will only cast Overgrowth (Dummy) because it doesn't have the Entangling Roots ability. If it would have both abilities, well, that would bug
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.