This is a really strange bug in 1.1.1, probably hasn't been reported before and likely still present in 1.2.0.

What happened:

I was a crowned champion of balance, alignment N-. My piety was positive, but not unusually high.
The altar was grey.
I sacced 100.000.000 goldpieces.

The following happened:
- the neutral god started a fight with the lawful god
- the lawful god won and the altar was converted to L
- my alignment changed to C- (-10000, according to a blessed scroll of balance)
- in spite of that, I did not become a fallen champion, but converted to a champion of chaos!!

I kept a copy of the savegame just after all this happened, in case it's any use.