teemo can instead Prock Thunderlords with one auto.

It makes sense. You have the actual auto attack, the on hit damage from his e, and then a tick of poison which amounts to 3. Enough to proc thunderlord's.

It's confirmed as a bug and a fix is already created and is coming up for Patch 7.1

If you want a source you can search through the Red posts 2 or 3 days ago.