I have, on multiple occasions (across the LG G Pad 8.3 and the 2nd gen Amazon Kindle HD) encountered enemies that I could not target.
This particular shot is on floor 28, Mythic 3. My hireling doesn’t target it, skullshield doesn’t target it, and I don’t get a target popup. Also, I have no idea why the Cartographer isn’t attacking or moving at that point. It and I stood still for many minutes as I chipped away at its health, hehe.