This is a minor bug, Purely graphical and hardly noticed.

Intended:
Viewing nametags from different angles shows the nametag rotated so its always pointing to the player. This is fine in First Person and normal Third Person.

Unintended:
When viewing a nametag on your screen in inverted third person (Viewing the front of your own char) rotating your camera up and down rotates the nametag the opposite way.

Possibly Minecraft not accounting for the inverted 3rd person?

Attached is a screenshot showing the nametag is pointing downward when the players head is facing upward. A direct inversion.

p.s.
I've done a quick search and haven't found anything like this. Correct me if this report is a dupe.