Haha.. the sequence of events should be

1. Bought the bike
2. Took bike for first ride and noticed fork clunk when going up hill. Got my back hurt too due to the aggressive negative angle stem.
3. Flip stem, when removing the top cap screw, it was very tight. Top part of compression plug was stuck with the screw. I didn't know it because i thought that's normal. (Never seen a compression plug before)
4. After flipping stem, fork still clunk. Decided to disassemble headset. To my surprise, the top cap screw detached from the top part of compression plug. This lead me to notice the big gap between the steerer tube and compression plug.

So I'm pretty sure i didn't mess things up. The top cap screw was overtightened in the shop thus pulling the compression plug up. 🤩

Now after i hammered the compression plug flush with the steerer tube, the stem gap appears to be too much (~6mm). Isn't this obvious the factory mess something up?

Note: I cannot bring the bike back to the shop due to covid lockdown.