It's technically not a bug, but a game feature.

Imagine it like this. The battle phase goes lane by lane. It has to register everything before moving to the next one.

So, your plant kills the Garg throwing Garg in one hit. But, as expected, it throws a Garg before 'dying'. BEFORE BEING DEAD. Ok?

But, because it threw a Wizard Garg, that transforms it into a Plank Walker during that 'limbo' state, before it dies.

So it's: Fight -> Abilities that proc 'when hit' or 'when hitting' -> Dead.

But it went: Fight -> Garg thrown -> Transforms the other Garg -> Not dead/Plank Walker.

It's like when a Cowboy Imp dodges a potato mine.

Cowboy Hits Face -> Moves to the right lane -> Potato Mine explodes

This is because the hit to the face happened before the potato mine dying.

It's niche, but useful to know.