Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32h5/STM32H523/FDCAN/FDCAN_RXF1A#0x0
FDCAN Rx FIFO 1 acknowledge register
Rx FIFO 1 acknowledge index
https://github.com/modm-io/cmsis-svd-stm32