Mini 3 Battery compatibility?

Can someone please give me a reasonable explanation to why the Mini 4 Pro battery will not function in the Mini 3 while fully operational in the Mini 3 Pro?

M3 and M3P are fully interchangeable. M3P and M4P too. How come they left the M3 out of that equation??

Fun fact: the M3 will propose a battery firmware upgrade for the M4P batteries. While the M4P can’t find any updates. I’m confused.