Unit upgrade bug?

[Fanatic Demon]

I tried upgrading my expererience warior unit to a axman unit. However something strange happened. After upgrading, I suddenly had 2 units, a new axman unit (without any experience and bonusses) and my old warrior unit (with old experiernce+bonuses). Question is, is this notmal, or is it a bug?

 

[Mercade]

This is not the way it should be. Is this something that can be repeated. Did it happen before, or did it happen again? Do you have a save-file from just before the bug happened so that people can verify and see what happens?