Hi All,
Seconded, I've just encountered the same bug during the first level of Svriir Cave. The creature named "Caved Noxious Swarmwing" is immune to targeting with autoattack (left click) and every other attacks - including AoE skills like Phantom Blades. In addition, the creature's HP bar is not showing while the cursor is placed upon its body, however, it is displayed if the cursor is aimed slightly next to the creature (targeting is still not possible in this state).
The creatures just follows me through the dungeon. :)
I am using a simple white pistol in one hand and no other item in off-hand.
Please investigate! Thank you!

Hope this helps in debugging:
Skills that work against these un-targetable creatures
- Wings of Ishmir
- Slayer's Flurry
- Evasion

Skills that DON'T
- Gunslinger’s Brand
- Stings of Krearion
- Phantom Blades

Also for some reason staff weapon works against them. It looks as if the staff projectile can recognize the creature unlike pistol's / bow's

I am using staff weapon to nullify these random threats