Got my fresh Bela yesterday and got it on my test bench today.
It does seem like my old Bela has some issue that the fresh one does not. They both exhibit the issue I noted earlier where powering from VIN and grounding via a System ground (i.e. P1.16, P1.22, P2.15, P2.21) while a USB A cable is plugged in prevents the board from powering on / booting up.
Strangely - If I use the Analog Ground (i.e. the ground pin socket adjacent to P2.17 on the ADC row) with VIN - my fresh Bela will power on / boot without issue with or without a USB A cable. My old one will not...So, this solves my problem for now.
I will also note this is only an issue using the voltage step-down. If I use an apple USB charger with a cable stripped to bare wire on one end and just plug the 5V and Ground from that into any ground it will power on / boot. So, it is somehow related to the step-down...but I'm not entirely sure how. If you've got any thoughts there I would be interested in hearing what you think the causation might be - but regardless, I think this is officially solved for me.
Thanks again for all of your help!!