The last area I cleared as of him evolving was 36. I don't remember how many effort hearts were filled, but I'm not sure how much that factors into it. Other than that I don't recall anything particularly unique.
Aight I found the logical fallacy! I was working under a false notion that if a Digimon exists on both colors, and it evolves into a Digimon that exists on both colors, that the outcomes would be the same. When testing evolutions for every Digimon being unlocked, I never ran into a case where that wasn't true, and so when it finally came time for me to test evolutions for fewer areas unlocked, I tested evolutions that were the same on both devices on just one device. As such, since Mametyramon would become Chaosdramon X on both devices with area 45 cleared, logic dictated that with no areas cleared it should still evolve into the same thing on both devices, but NOPE! That was incorrect! Mametyramon becomes Prince Mamemon X on Red and Metal Piranimon on Purple.
Glad that mystery is finally solved, thank you for reporting the error!
Also this continues Red's dominance over Purple, lol. You only have one way to get Prince Mamemon X on Purple, and now there are THREE on Red!