This may be a feature request rather than a bug - I'm not sure if this is supposed to work but isn't, or just isn't done yet.

I'm not sure about hostile mobs, but definitely passive mobs suffer from not seeing carpenters Barriers, and the barrier Gate, as blocking terrain. I've tried using the barriers as chicken pens and cow pens, and both animals get even more hung up in the Carpenter's barriers (and gate) than they do in the Vanilla fences and gates, as they try to path right through them.

This is demonstrable via the following experiment:

1. Create a small (5x5) pen with 3-block high walls (dirt, obsidian, whatever), using one Carpenter's gate as a door.
2. Spawn 15 chickens in the pen - enough to induce a population pressure problem.
3. The chickens will crowd in the carpenter's gate, "seeing" it as a hole in the barrier rather than as the solid obstacle that it is.

It may be the same as the door situation, where AI is hard-coded to to see certain blocks. I haven't looked into the gate or barrier situation specifically, though.