Ah, I can duplicate this behavior too. I think it is probably related to emacs being a metapackage. It does not include emacs itself but forces the install of emacs-gtk. In my mind removing the metapackage should allow you to autoremove dependencies, but people have broken their systems badly with this behavior so it may have been changed or it’s stuck behind some configuration option.
Removing emacs-gtk itself will work as you expect. You can also install emacs-nox for a cli-only one that is smaller.
Edit: there is a setting called APT::Never-MarkAuto-Sections that by default includes meta packages and I think is the cause of this.