Because it’s not related to the PCB but to the CPU drivers.
On one hand, the i.MX 8M Quad silicon issues meant not only that the temperature of the battery was way too high, but also that it could discharge too quickly. On the other hand, the i.MX 8M Mini is made in an updated silicon process (14nm instead of 28nm) and has reduced features, resulting in a lower power consumption–but it’s new, and there could be unknown issues and risks. So a lot of research was made, and our development team started evaluating the i.MXM8 Mini to see if it could be used within our requirements–free software only, no binary blobs, mainline based software stacks.
And then the month of February began, and something else unexpectedly happened: NXP released a new software stack for our first CPU choice, the i.MX 8M–and all of the power consumption and heating issues suddenly disappeared!