STM32L4 CAN FMP remain 0 for large data chunks

Question

I have CAN application where in remote device following sequence of packets

1st packet : 8 bytes Remote sends : 8 bytes

2nd packet :8 bytes Remote sends :Ack Remote sends :63 bytes .

So the ACK and 64 bytes are one after the other . I can read the 8 bytes data when I get an indication that there is data in FIFO . To read the next 64 bytes , though there is data in buffer , FMP bit in CAN_RMR register shows 0.

Pls point me in the right direction or any documents I can refer from STM .

Thanks