Amending a signed commit overwrites author

Amending a signed commit overwrites the author (bad) and the committer (good).

Amending an unsigned commit does not overwrite the author (good), but it also does not overwrite the committer (bad).

Check out a branch where the latest commit was authored and committed by someone else
Run git log --pretty=full -n 1 and verify that the Author field shows someone other than you
Stage changes
Open the commit dialog
Press Ctrl-A to make the pending commit be an amend operation
Press Ctrl-D to finish the commit
Run git log --pretty=full -n 1 again
Observe that either the author or committer is wrong (depending on whether the commit you amended was signed)

Amending a commit should overwrite the committer, but not the author.