Backup /home/yourusername/.config/dconf/user file just in case things break later.
In a terminal: gsettings reset org.cinnamon.desktop.default-applications.terminal exec-arg
Open dconf editor and see if the exec-arg entry is now visible.
If not:
Close dconf again.
Terminal: gsettings set org.cinnamon.desktop.default-applications.terminal exec-arg "--"
Open dconf editor and see if the exec-arg entry is now visible. Maybe it doesn't show up if there's nothing in it (which is strange, but I guess possible).
Also "--" is usually a safe command line thing because it means "end of arguments" for most commands.
You can try:
Terminal: gsettings get org.cinnamon.desktop.default-applications.terminal exec-arg
...to see if the entry has the double dash or not, whether or not it shows up in the dconf editor.
Further things:
Uninstalling and reinstalling the dconf editor might be worth a try.
The gsettings command should be usable to set exec-arg regardless of what the dconf editor says.
Restore that backup if things do go screwy. I can't see any obvious reason why they would, but I can't explain the dconf editor's behaviour either.