I'm running an evaluation with CAN configured for FD at 1Mbps for arbitration phase and 2Mbps data phase. I'm generating traffic from PCAN-View on a PCN-USB X6 with 16 bytes data at 1msec intervals (at this rate some messages are dropped). Inside the IRQ handler it reads from the mailbox a DLC of 0 instead of 0xA, which I caught by adding an assert at that point. If during the assert I look at the MB register again it contains a correct value of 0xA.
Looks like the interrupt happens before the mailbox is up to date.