Subaru WRX cold start misfire

Hi everyone, so my nephew's Subaru WRX that he bought not too long ago does this misfire.
Car has 100k miles, BONE STOCK (besides Axle back exhaust)
Automatic
(Could've been modified before with cobb)
Only happens when cold / first start up the day,
Only does this for 30s - 1 minute
Spark plugs been changed
Once it stops the car runs fine and drives fine.
No check engine codes, only threw a misfire once on a start up.
*Checked coil packs, wiring, fuel lines, intake / intercooler for leaks. Ecu wiring, and the only codes I got was a p0300 nothing else.

I'm really thinking that this has to do something with the tune, but I'm not the Subaru expert. Tried googling these issue but nothing helpful popped up.
What do you guys think we should look into?

Thank for the help! :)