Hi,
four of my xenonauts have been injured during the first mission. They were about 70% HP so I could send them to the second one, and when the mission began, they were still harmed. Thus I was able to fully heal then with the standart medipack, and when the mission ended, they (every four of them) were 100% life.
In a nutshell, if you have injured xenonauts, send them, heal them at the beginning, kill'em all, and you will be full again. No need to wait, no need to medic, just riot!
I... believe it's a bug, isn't it ?