RP2040s don't boot if Mega's 5V rail comes up first

I think there is a back powering issue that is causing the RP2040s BOD to disable them?

In practice, this is not an issue since the Mega supply is controlled by the RP2040s. 

I also suspect it is one of the buffers that are pulled-up to the Mega 5V rail while connected to the RP2040 5V rail. (maybe?)