I will start this off by saying that I'm not hating on Tahm, honestly he is pretty fun, but seems to have a few glaring bugs.
I decided to run a 1vs1 with a friend of mine as we had both recently purchased Tahm with the post release week price drop, so we load into customs with the usual trash talk prominent in my group of friends, the "git guds" and "step up feggits" were flying like birds through the room. So we get into game and I decide to actually read his abilities, settling on devour first because it sounded funny. I manage to get three passive stacks on him about the time the minion wave shows up, and when I go to devour him it grabs a creep on the other side of my character model. So now we decide that the 1vs1 can wait, and we need to science a little.
From what we discovered there are a few strange interactions and inconsistencies. For starters devour will always prioritize minions, you can auto attack a champion for a solid 15 seconds and when you try to devour if there are creeps nearby it grabs them instead. Dang it Rito, when I hover over a champion it's because I want to ruin their day, not eat their creeps.
Weird interaction number two, Tahm vs Tahm sometimes allows one to use the other's tongue lash to devour from range, namely it lets one Tahm eat the other from maximum tongue lash range if the passive is stacked up properly.
Planning on posting this to the forums in a bit, but if anyone else could give these a test i would appreciate a few extra opinions on the matter as I don't have any video of these interactions, and it could just be that I read what was happening incorrectly somehow. Thanks for your time, and have a good one.