Hello,
We've been told that this ninja is supposed to work this way, though we haven't been told why. It's definitely an odd interaction that goes against how barriers normally work. But he also has the potential to poison your own team if you use deep forest wall on yourself, too. Definitely an odd ninja.

Edit: I'm also being told that the barrier casting is apparently based in his personal initiative, not the team initiative, so it's heavily affected by his position in the team, i.e., whether he's p1 or in the support position.