Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32U599/FDCAN1_RAM/FDCAN_RXF0S#0x0
FDCAN Rx FIFO 0 Status Register
Rx FIFO 0 Fill Level
Rx FIFO 0 Get Index
Rx FIFO 0 Put Index
Rx FIFO 0 Full
Rx FIFO 0 Message Lost
https://github.com/cmsis-svd/cmsis-svd-data