Often, the atgm hits the era, explodes on it and rather than activating it and making use of the tandem warhead, the metal jet just sails right through without any activation as shown in the clip.
Its happened to me a couple times but now they have been more frequent, sometimes KE rounds make a hole in the era plates, clearly visibly in the area it should activate, but will refuse to do so and so provides no protection.
It may just be the hitbox of activation area around the era is not accurate, and missing out some key areas of era that should activate if hit, but it happens much more often than if it was a normal bug.