Supposedly the following command should disable that "feature":
gsettings set org.cinnamon.settings-daemon.plugins.xrandr active false
EDIT: apparently not anymore, but there's another solution:
https://github.com/linuxmint/Cinnamon/issues/6646EDIT2: per this PR, you should be able to disable it in the configuration panel: https://github.com/linuxmint/cinnamon-settings-daemon/pull/1...