I'd noticed with my PHEV'18, when driving in cold weather (let's say -15c) on long trips, if the traction battery is somewhere between 20-60%, the ICE will stayed on most of the time in Normal mode, whereas in Eco mode, the ICE will go on-and-off periodically. I also noticed for regular driving...
This happened to my new 2018 Outlander PHEV as well, after picked it up from the dealer, drove it a few times. The dealer updated the firmware and so far so good for two months now.
I also noticed my 2018 Outlander PHEV driver side rear passenger floor was very warm after returning home from a 200 KM drive, were you able to get an explanation from Mitsubishi?