According to Tire Guides, there was no change in tire pressures from 2018, 2019, and 2020. For a BMW i3. But they don’t delineate an REX model and maybe that is the difference.
Normally when an OEM increases inflation pressure, they are doing it because the weight increased, or they have a handling issue they are trying to solve, or something like that. In this case - like Mustangman said - I wonder if there was a tire size change. There has been a lot of activity lately on that front - new sizes, new types of loading, etc. I doubt very much anyone here or outside of the BMW design department would know.