Hello, if I understand the issue correctly, you do not see a reason why command 0x10 returns code 16 in the example application.
The command 0x10 is registered for a callback function:

So the function "my_appcmd_handler" gets called automatically when FreeMASTER probes the command execution. The example code of this callback is quite trivial:

Which is the reason you get the return code 16 (=0x10) as answer.
I also confirm there is an issue in FreeMASTER with displaying a message for unknown responses as you reported for app.command with Id 0x02. The response 0xcd should be reported as "unknown response" but generates no message at all.
Thanks,
Michal