This is nothing to do with Reality actually, you might want to post a bug in the dayzmod github for this. However, I am pretty sure this was an issue in 220.127.116.11 as well, I remember more then a few times having seen something similar.
Did you shoot the window frame itself or the window? From what I understand its better to hit the frame and to hit the windows on either end. I know hitting it with an axe is usually better then shooting it as well.
I was shooting across them so may have hit the frame. We also had a dog each, as were testing piXels dogs.. Not sure what finally broke the window. Is interesting that window breaking is client side.. There were zombies about too, maybe they broke one to get to freshly wrapped (bandaged) meat in the deli..
As thevisad said, this is not a Reality related issue. This has been an issue in DayZ for ages. As said, it's because it is a client-side mod and it is rendered client-side and then sent out to the people that are around the hospital.