2013/11/29 Wolfram Wagner <wolfram@wagnerw.de>
I wonder, if this is so by purpose or if this is a bug...

The "-MOD" suffix makes me think that this plane is a somehow altered version, so it's hard to say what might be wrong there.  However the official tu154b uses standard FG transponder implementation and doesn't alter altitude values generated inside it in any way, i.e. the values come from FG itself, tu154b code is not involved in their production in any way.  Perhaps it's possible that the person behind that plane used some FG version that had a bug in transponder code that generated wrong values (or there's some bug in OR).  Otherwise I don't get how the problem could happen.

  Tomash Brechko