I've experienced this too, but consider it operating as intended for the reasons stated above.
If you'd like to experiment with it more directly, try switching to MM before EV range depletes fully, the gauge will show a switch to CS mode but if you switch back to normal it will revert to CD mode showing electric miles (and depending how long you stayed in MM, probably more than you started with). However, do the same experiment after fully exhausting the EV miles and the gauge will never switch back to CD, no matter how high you raise the SOC with MM. Finally, fully drain the CD range in normal, then run in MM for 10-15 minutes, then switch back to normal, you're now running in electric but recording miles as CS. Immediately turn the car off and back on again and it will switch to CD and record miles as EV.
CS mode does not equal gas as power source, it just changes the algorithms that control from which source the car will draw energy under different parameters.
I dream of a better world, where chickens can cross the road without having their motives questioned