I've not noticed this. And I've been getting a good fair share of injuries lately.

One thing that I have noticed is that my Major injuries seem to never heal. I was under the impression that Minor and Major injuries were supposed to heal themselves, but Critical needed to be manually healed. In my case, through 1.5 playthroughs, I've never had a Major injury that's healed itself, i've always had to heal it. I may just be mistaken about the mechanics though. Maybe it's only minor injuries that heal over time.