Hi,
MB flag is set when message is successfully sent or received, regardless of payload.
What did you mean by "flag is set up wrongly"?
How do you manage the MBs for payload of 16/32/64 bytes? Note the MB's address offset changes for different payload in memory block, depending of MBDSRn setting. Tables 53-15 to 53-17 summarizes this.
BR, Petr