About 2 years ago I posted here about my 2018 Subaru Impreza which was emitting smoke at start-up. The car had no other obvious symptoms and the dealer told us it was condensation, but we knew it was not condensation. I got many excellent suggestions here for what it might be and we followed up on them but nothing solved the problem. I’m posting now because we finally think we have figured it out. We continued searching for ideas about what it might be and we found some Subaru owners posting that their cars emitted smoke when they were parked slanted down or to the right (passenger side). My driveway slants and my car was usually parked slanted down and to the right. We believe that the problem was that the boxer engine allowed oil to seep into the combustion chamber when the car was parked slanted down and to the right. The fact that the dealer overfilled the oil by about 1/4-1/2 quart when changing the oil did not help. We are now parking the car so that it is level or slanted up. We have not had the problem since we started paying attention to how the car is parked. I hope this is helpful for other Subaru owners who might be having the same problem.
Thanks for the update…
Here is a link to the original post about the issue…
So the thoughts are that:
On this “Flat Four” that oil from an over filled crankcase is collecting behind the piston and seeping past the piston rings, or the rings are not swiping/wiping the oil off the cylinder walls during start up.
Or the Stem Seals on the valves are not up to the task (hardened or cracked) and are allowing undrained oil from the heads to weep past them and into the combustion chamber.
And by parking the vehicle inclined differently, the oil drains back in to the crankcase more efficiently which solves the problem… 
Then remove a spark plug from offending cyl and insert borescope and see if any oil is in upper cyl after being parked overnight.
But it is possible.