(OSD3358) UART0 is receiving characters when there is nothing connected to it

Custom board based on Beagle Bone Black using Octavo Systems OSD3358-BAS. UART0 port is wired identical to BBB. UART0 is receiving characters when there is nothing connected to it.

Some sequences of characters are very similar to characters that were recently sent out the UART0 port, some are seeming random and on some occasions it receives a serial BREAK which causes the next character to be treated as a system command character. Unfortunately, if the next character is a ‘b’ then the system will reboot.

What could be causing the unwanted characters to show up and what can I do to stop it?

Thank you,