This happened to me more than once:

1. I was writing a long commit message (without clicking the describe button)
2. While writing, I noticed I forgot something and made a small change
3. When going back to gg, the commit message was gone and I had to start from the beginning.

My expectation is for the commit message to stay there, even if the commit I want to describe changes (at least as long as it's still the same underlying revision.