Alright, I got a few of my buddies together and we did some testing with it. Same map spot, same scenario (me on Reinhardt, and my friends on Bastion and Mercy). First I checked if it was a problem with the stairs (someone on reddit said recently that Earthshatter didn't work when used on stairs), by having my buddy face the stairs, while I Earthshattered. It worked, so we continuedRJ 17 said:Snip
I then tried to see if line of sight was a problem. First I tested on the wooden pillar, and when I ulted, The bastion didn't budge (I was looking directly at him, and I could see roughly 50% of his silhouette). However, seeing as my friend couldn't see the point, we supposed that's not where the Bastion was. So we put him on the other side of the stairs, close to the spawn tunnel. As it turns out, even if I could see 2/3 (his chest cannon and his folded left arm) of the Bastion, my Earthshatter would not have any effect on him. he had to be almost fully visible for me to stun him (which did work). This setup (Bastion at the end of that spawn tunnel overlooking the point) is is generally used on that map.
I think what you saw was a Bastion that wasn't quite enough in your line of sight. part of his body must've been hidden behind the wall, and when you Earthshattered, the game must've considered him not in LoS. I didn't record the experience (because I'm stupid like that), so I invite to to recreate it yourself (and it's a good opportunity for you to check that you can stun Bastion with Earthshatter).