It may be that if you change the setting to Add but GIMP crashes, the setting isn't being saved on exit. If you open GIMP, switch the setting, close and reopen, is Add retained?
As for crashes, please feel free to report the stacktrace on our bug tracker if you haven't already: https://gitlab.gnome.org/GNOME/gimp/activity
We're trying to fix them all, but some are difficult to reproduce without more info (like, some bugs only affect Flatpak, others only on macOS, etc).