Mountain Survival in Civilization 5

I made an experiment with two knights both having medic1 + medic2 + fountain of youth upgrade. I then moved one of them on top of a mountain inside my territory and the other one next to it on a non-mountain tile. The knight landing on the mountain takes 50 damage and ends it's turn at 50 ( 100 - 50 ). Then it heals 40 before the end of next turn and takes another 50 damage after the end of turn bringing the total to 40 (50 + 40 - 50). Thus the unit seems to loose 10 hit points per turn. I have no idea why the unit heals only 40 points per turn. Perhaps fountain of youth promotion and medic promotions do not stack at all.