For this episode, the Forest Field won't be added back- the full episode is already released. Maybe in 16, if there's reason to. Depending on the difficulty of the fight that others have with PULSE Tangrowth, it may be added again.
Furthermore, because the battle is fought on the new overgrown Obsidia Park, it must share the field effect with the rest of the park. The only trees that exist are the artificial ones created by the Tangrowth. In comparison to other fights in natural forests in Reborn, the effects of this field wouldn't be the same. The reason for adding a field effect would be to make the fight a challenge, but it should make logical sense.