I'm unsure when this took effect, but after one update (I believe it was 1.9), snowballs no longer apply the HurtTime tag to a player on Vanilla servers, but still apply that tag to mobs. This causes methods of using snowballs as hit detection to become far more complicated, as detecting the HurtTime tag was the simplest method. Since players rarely use snowballs outside of heavily modded servers, this issue has likely gone mostly unnoticed by anyone outside of the command block community. My best guess is it has something to do with the fishing rod changes that occurred in 1.9 that made fishing bobbers no longer "hurt" players, and maybe snowballs use a similar line of code, if not the exact same code.

I first discovered this bug a while ago while coding a minigame involving fighter jets in 1.9, but didn't report it because I thought it was a localized bug. However, I discovered this Reddit post (https://www.reddit.com/r/MinecraftCommands/comments/2gst8q/snowball_hit_detection/) tonight that confirmed my suspicions that it was not specific to just my friend's server.

Please fix this issue. Though it is a smaller issue, it would help me and many other command block users a lot.