In the Bug Report, it says if you have a way to reproduce this bug, get in touch. So I'm here, because after expirimenting with my Wolfkin coach, I think I've found one way. This coach, I had training in the Tow drill. (++Lif,+Pow,-Int)
I replaced it with a Kelmadics after a while, as it had better stats. The Wolfkin coach was still active, just not doing any drills. I got the retirement warning for it, and it passed, and I thought the Wolfkin was gone. After some more time, the Kelmadics coach got it's warning and left. But in it's place, the old Wolfkin coach took it's place in Tow. It doesn't show up for other training at AGIMA, and I can't really do anything with it except Tow training. I invite others to try this method. So you don't have to read this message again, I'll put the instructions below:
1. Get the coach you want to stay
2. Assign it to a training (more than one if you like, I don't know if it will work that way, but try)
3. Use it for a while, but before it's retirement, get a different coach to do it's drills (all of them)
4. Use the other coach until it retires (you should have also gotten a warning for the old coach's retirement)
5. Check back, and hopefully, the old coach will have resumed it's old training.
Okay, there. Hopefully this will be the way it's done. It worked for me.

This is pretty much how it worked for me as well. Seeing the whole thing written out makes me think back to what happened. It went very much as described.

-Thorr