I was trying to complete a title reward by enhancing a weapon to +35 as a Hunter (with Ranger as the sub) with Assault Rifle as the weapon. However, instead of getting Hunter's specific title, I got the gunner instead?

I'm not a native English speaker and my interpretation of the condition is "as a Gunner". Not "Enhance a 13* Gunner's Weapon to +35" Is this a legit bug or did I miss something? Either way, I still got my SG and the title reward.

The text of those Titles is poorly translated. The Titles are linked to the Weapon types the class uses, not to the class you are using when you enhanced it to +35, so it definitely should say "Enhance a 13* [Class] weapon to +35."

To get the Hunter one, you have to enhance a Sword, Wired Lance, or Partizan.