Max,
This is a difficult problem to logically understand unless your Mark I & Mark II Beo4's have a fault and the same fault.
I took a Mark II & a Mark III Beo4 and triggered V.AUX and noted the code sent via the IR of both of them and it was
0001801000130000000158 for both remotes which is correct. Also the code for V.AUX has never changed since day one.
For test purposes why not swap the cables of STB1 to STB2 so STB2 will now be triggered by SAT/DTV