I've never owned an OLED device until now. So I have no personal experience with older iPhones, or any phone with an OLED panel prior to the 12 pro max. I am simply going by my experience with display tech in general and how I understand things to work, based on my own experience with displays I've used, and what I have read.
I can tell you though, while LCD and OLED are two completely different technologies. When I calibrated a Samsung monitor to cinema settings it was much more warm looking then any of my other displays with the same calibration. That's just comparing LCDs, and an OLED wasn't in the mix then. The factor that remains the same here though is in my case both warmer displays were Samsung, and I have read that they tend to have warmer displays then other manufacturers. Samsung Makes the panels for the iPhone 12 models. The displays may have looked different on older phones because they were either LCD, or an LG panel instead, not to mention any changes Apple may have made to their calibration specifications for the new phones.