Hmm. It doesn't seem to be quite fixed.
My follower definitely hits harder now, and takes a lot more hits before he dies, but the numbers are still lower after a relogin.
I just raised my follower from level 17 to 18, and his hits on Furious Bears went up from 68 to 85. After a relogin he now hits 70 on the same bears. So, it did not go all the way down to how it was before he leveled up, but it did drop rather drastic.
It happened again from level 18 to 19. This time his hits went up to 87 after leveling up, and then down to 70 again after relogin.
Also, as I have noted before, something seems to happen halfway to the next level. He began hitting 75 (instead of 70) halfway to level 20. This time I am absolutely sure that nothing else had any influence on this, like leveling up skills, and it seems to happen exactly at the 50% XP mark.