Is the RQCPx set at the end of the EOF duration, or IFS duration? The reference manual state diagram shows Bus Idle as the point when RQCP is set, but exactly when Bus idle is determined and the RQCP bit set is not clear.
The issue has to do with using a single mailbox. If it has just completed the transmission of the message, is it possible for the TX interrupt servicing to load the next CAN msg into the mailbox before some other CAN node has started sending. Using multiple mailboxes solves this issue, but opens up a number of other less tractable issues.
