I rarely comment on "RIP" topics, they are commonplace in this hobby. Sometimes the species has a short lifespan naturally, more often than not it is due to something in our care, other times you could be the best damn keeper out there and bad luck just finds you. I'd lay my money on that in this. However losing two to the same odd hole brings out my forensic training. Also after keeping these animals for so long, I learned a long time ago to always learn what I can from any death where possible.
Jan you're a Parson keeper, I'll always help out a fellow Parson keeper. I am soooooooooooooo sorry for your loss since these animals are more than a common pet, they become an individual and therefore a family member.
Despite that, push onward.
There has to be something consistent that would create the same type of injury. I highly doubt it is genetic, unless the animals came from the very same parents and even so, that issue doesn't seem a genetic one. Could it be from something acidic? Could it be from some kind of feeder? These thoughts guide me as I work my way down the rabbit hole of any problem that arises with my pets. While I was out of state for seven days this past week my Wife did a fantastic job taking care of my dozen reptiles. Despite that I lost the older female quad I adopted named Quadralupe. The wife was upset but she didn't do anything wrong. The animal was eating two days earlier and was found in an nearly upright position on a branch in a porthos (quite unlike the typical on the ground and dead as a door nail position we see more often). It looked like the animal went to sleep in her sleeping spot and just expired. My best guess is age since I know that animal was 6-7 years old.
While your loss is raw, painful even, and you may want to withdraw from the hobby, push onward if you can. There is an answer, but you won't find it unless you break everything down piece by piece by piece just as I do in my career and my hobby.
Very sorry to hear of Cooper passing.