So this seems like either a bug or a weird interaction with enemy targeting. When using the bulwark shield if an enemy target an ally standing behind me after I have placed the shield (using the alt attack function) the enemies can just clip through me. It seems to be mainly elites and bosses that can do this, pox walkers are blocked off as expected. It feels like it should not work in this manner and is particularly frustrating on higher difficulties.
Had this happen to me repeatedly as well, and I am not a fan.