Easy solution to the current problem:
* Medkits only give Temporary HP, which will only last until the end of the mission.
* If your health gets to 1HP you can indeed heal back to full health, but when the mission is over you get back to 1HP.
* You can only get one Temporary HP for each lost HP. That prevents soldiers from taking damage, healing, taking damage again and healing again, endlessly.
* If you want to complexify it you can make the Temp HP wear off with time, like pain medicine losing its effect.
Explanation:
Field medicine is not brain surgery, it involves cutting a bleeding and avoiding the victim from passing out. It's basically bandages, plus morphine/adrenaline or such. So, a medkit should never heal the way the hospital and time does.
When a unit gets injured, you can treat a wound so it won't bother for the next hour (but will have to be treated in the long term), and you can treat his nervous system so he doesn't feel pain, and/or gets stimulated (but pain will come back in the long term, as well as fatigue).
If you are wounded, treated with a medkit (treated, not healed), and then get wounded again, the chances you can get effective treatment are abysmally lower, since you are adding new damage to previous wounds.