Can the watchdog on an STM32F373 trigger a reset while the RESET pin is held high?
No. The nRST pin of STM32 is not meant to be forced high externally, because it is bidirectional open-drain IO pin. Simplest way to avoid this is of course to set the RPI pin as input and only set it to low and output when resetting STM32, and back to input to release the reset.