Take a look at the replay I recorded and uploaded to Youtube showing the issue twice: Once in full speed , the second time in .25x speed
It looks like when Fizz interacts with Sheen it allows Thunderlord's to proc multiple times ignoring it's cooldown. I believe this is because of Sheen. For instance, in the video you will notice that the purple thunder coincides with Sheen going on cooldown.
Maybe it's because Sheen is the 3rd source of damage causing TD to proc. Because of this, maybe the game does not put it on cooldown because it did not come from a source of damage from Fizz.
Although this observable synchronization between Sheen and TD makes a strong case for Sheen being the culprit, maybe Fizz's passive W which applies Damage Over Time might make this a more complicated matter.
Steps to reproduce:
Proc thunderlord's with W empowered auto attack OR Q. (i.e finish the combo with either of those moves)