Max over heal number flickering?
While constantly healing someone, the health of that person increases. However, if you over-heal them, the number will start to decrease until it goes back to the maximum life. You will always see the number fluctuate between the 150% over-heal and the number below that. What you are seeing here is normal, not a bug.
As @Toomai said in the comments below, this isn't seen in low-health classes such as scout because their over-heal decays slower.