Hello @watchforstock
looking at the ATOM RS485 schematic the only way I see which would keep the M5Atom from powering up when plugged into the ATOM RS485 is if somehow the 5 V line is (partly) shorted to GND.
Now, there is a built-in DC-DC converter in the ATOM RS485 converting the input voltage from the RS485 connector to 5 V. In your case however this DC-DC converter is not fed an input voltage. Maybe the DC-DC converter IC didn't like the fact it was fed 5 V into its output from the M5Atom.
You could try to measure the 5 V on the Groove connector while the M5Atom is plugged into the ATOM RS485 to see if it somehow is pulled down.
Thanks
Felix