I've noticed when I kill looters and any other human enemy I'm not granted exp. Is this a actual result or bug for XB1?

Iirc, you only get a small amount of kill xp, and only until you fill up the bestiary page for that creature. After the page is full you get bupkus. Its a "feature" to "prevent" players from going murderhobo on the game.

Or put another way, to not punish players who don't want to 'farm' monsters for two weeks straight for a few levels up here and there.

 

Combined, you still get a pretty big chunk of XP from killing monsters / filling up the bestiary, but you also get loads from finishing quests (and there is plenty of XP in the game anyhow).

In particular, since the OP specified human enemies: no you don't get any xp from killing human (or any other kith) enemies and this is intended. In fact you don't directly get xp for killing non-kith creatures either, you just get xp for completing their bestiary page which happens whilst you kill them (but there's a limit to this).