The current CAN bus driver for STM32 doesn't support fifo selection when reading incoming messages. It's reasonable to conceive of a scenario wherein message destined for fifo0 starve out messages for fifo1.
Although it seems to be abandoned now, the fdcan crate from Richard Meadows, which seems to serve as the inspiration for this implementation, had split up reads into receive0 and receive1 which I think gives the user much better control.
I've put together a quick PR that restores the granularity but prefers to receive an enum to the read function specifying the desired fifo instead of having separate functions.

Please let me know if this feature is desired and and I can finish fixing the examples.