I have the same problem with my 2014 Legacy and it is NOT due to warm up or tire pressure. I never “warm up”, in fact I don’t even wait the 5 or 10 seconds unless it happens to take me that long to put on the seat belt. I am a tiny bit, just a tiny bit I tell you OCD about tire pressure. Don’t get me started.
The problem, at least in my case is the CVT transmission. Below 45 degrees F, it holds the gear ratio lower, causing the engine RPM’s to be 2-400 rpm higher. It’s like a cut off, 46 and it is normal, 45 and the engine revs more, even after it is fully warmed up and it cuts the gas mileage by about 10%, from 30 mpg to around 27 mpg.
When I get on the highway, I shift to manual and upshift to 6th range. It’s always in 5th when I go to manual. For some reason, after I do this for a couple of minutes, it stops hanging in the lower ratios even back in auto and back in town. BTW, your CVT is programmed differently than mine as in 2015 they went to simulated gear changes, but it might still have delayed shift points in cold weather.