Solved
BOOT0 on STM32WB1MMCH6TR
The STM32WB1MMCH6TR module has a BOOT0 pin that is pulled high on the MB1868 "B-WB1M-WPAN1" board through (what appears to be) a non-populated resistor.
Without this resistor the BOOT0 input to the module is floating. Does the module pull the signal down internally or is the BOOT0 input to the STM32WB15CC on the module left floating?
Is there a design guide document for using the STM32WB1MM? So far I've been using the bare chip documentation and the B-WB1M-WPAN1 board as a guide. I do have the STM32WB1MMC datasheet (DS14096 - Rev 7 - August 2024) but there aren't a lot of details.