This could, in part, be due to the higher PWM frequency of the 13 Pro/Pro Max phones, compared to the 12 Pro/Pro Max ones, which didn't have ProMotion yet.
Linking my initial comment here, where I posted a table of the flicker percentage across a range of iPhones (
PWM flicker percentages), even though I didn't test the 12 Pro/Pro Max models, I did test the whole 13 range; as you can see, the 12 is in a much lower range than the 13 Pro/Pro Max, and could be a factor.
I'll be able to run the flicker meter across the 14 range in about 2 weeks time; I'll report back on what I find