When an entity teleports to a player, that entity receives its yaw and pitch rotation data regardless of if they are moving or stationary.

When either a player or entity teleports to a (this is not tested for all) mob or presumably any non-player entity with yaw and pitch values, the rotation data is only copied when the entity being teleported to is in motion. If they are standing still and simply looking around, it's rotation data will not be copied.

As far as I can tell, this seems to be an issue of non-player entity rotation data only being updated when they are in motion. I remember seeing a comment Searge made in another bug thread in which he mentioned that 'mobs don't serialise their orientation':

https://bugs.mojang.com/browse/MC-61587?focusedCommentId=191897&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-191897

Is there potential that this could be the issue?

Edit: u/TimMinChinIsTm-C-N-H from the MinecraftCommands subreddit (http://www.reddit.com/r/MinecraftCommands/comments/2raklr/teleporting_to_mobs_and_other_nonplayer_entities/) correctly mentioned that I should clarify that if you teleport to a non-player entity, the rotation data you receive will be the final rotation when they were last in motion. This is regardless of whether they have since rotated but remained stationary, meaning you can be rotated in a completely random direction unless you teleport as they are moving. Hopefully this better explains the issue at hand.