People are throwing out some pretty crazy ideas in this thread. Using isolation exercises vs. compounds has absolutely nothing to do with it and if lat injections were the cause you'd more likely see regression in the upper outer lats (like Nasser) than a space so far from the injection site.
Here's my theory as someone who has also lost a part of their back musculature.
I wonder if part of it is that we don't notice extreme damage to back muscles as much as others. At some point early this year I tore the lowest portion of the left trap around where upper and mid back meet. I never felt a sensation during training or noticed anything outside of usual soreness, but I started seeing bruising in that area when posing my back. Eventually the bruising cleared up but that lower part of the left trap is now gone - it's there on the right side, gone on the left side. It has no impact on strength, just an aesthetic thing. I'd bet it tore doing heavy Smith rack deadlifts as that area of my traps always got a lot of work on them.
Here's my theory as someone who has also lost a part of their back musculature.
I wonder if part of it is that we don't notice extreme damage to back muscles as much as others. At some point early this year I tore the lowest portion of the left trap around where upper and mid back meet. I never felt a sensation during training or noticed anything outside of usual soreness, but I started seeing bruising in that area when posing my back. Eventually the bruising cleared up but that lower part of the left trap is now gone - it's there on the right side, gone on the left side. It has no impact on strength, just an aesthetic thing. I'd bet it tore doing heavy Smith rack deadlifts as that area of my traps always got a lot of work on them.