Resolved! What are the correct FLASH page numbers for STM32G0 dual bank devices? It looks like there's an inconsistency between the reference manual and the Nucleo FLASH examples included in the STM32G0 Cube G0 package.
In the datasheet (RM0444) for 512k dual bank devices, it looks like the last page of bank 1 is 127 and the first page of bank 2 is 256 (see section 3.3.1)The flash control register (see 3.7.5) says, the valid values for PNB is from 0 (page 0) to 0x17...