Heya folks.
We just micropatched a temporary fix for this, so this should no longer happen.
What happened here in simplified terms is that the little legends are tagged in a specific way in the game as being "usable" in addition to another tag to make it not interactable in gameplay terms: "untargetable". Generally, being untargetable here means you can't really interact with it from a gameplay point of view and it's the reason why Cho'gath can't eat the little legends, and why yuumi can't attach to them.
However, the reason why the LL's are also tagged as "Useable" is so you can left click to interact/tickle them (They'll play a cute lil' animation).
So why did it happen? The three champions (Jax, Lee Sin, and Katarina) that could interact with them from a gameplay perspective have specific permissions to interact with units that have both tags (untargetable, usable). This is exactly what allows them to jump to Thresh's lantern!
Unfortunately, the least risky temporary fix that won't potentially mess with the three champions on SR involves disabling the usable game tag on companions, making it so you can't left click to interact with them, but fixes the issue.
We'll follow-up with a more permanent solution soon that allows you to pet your little legends , and without the bug.
Thanks for the patience everyone! :)