For anyone that still cares, here is a work around for this issue.

Here is the problem. Once you quit and exit Arma3 you no longer get an option to "Resume" your saved game, forcing you to start a new game. However the save game files still exist and may be used again. To get around this issue see the steps below.

1) Save your game using the "sitrep" option from your radio menu (press 0-8 in game).
Note: You may abort the mission and will still be able to load this save as long as you don't
exit Arma3. If you need to quit Arma3 and continue later continue to step 2.

2) Once you quit Arma3 navigate to the folder where the save game files are located. Example:
"C:\Users\your_username\Documents\Arma 3 - Other Profiles\your_arma3_profile_name\Saved\steam\DUWS%20Modified%3a%20RHS%20Escalation.Altis"

3) Copy or move the files in this folder to an alternate location on your computer.

4) Launch Arma 3 then start a new game of DUWS Modified: RHS Escalation.
-Generate a random mission by pressing the "Random" option.
-Leave all settings at default and click start.
-Once the mission starts, save the mission by using the "Sitrep" option (press 0-8).
-Now press 'esc' and "Abort" the mission.

5) Navigate back to the main menu in Arma 3 but DO NOT exit the game.

6) Alt+Tab out of Arma 3 and copy the files you saved (step 3) back into the save folder location
(step 2) and overwrite all.

7) Alt+Tab back into Arma 3, select single player, scenarios, DUWS Modified: RHS Escalation.
You will now have the option to "Resume" and continue your original saved game.

Note: Alternatively you may also save your game by using the "Rest wait/save" option by approaching the squad leader located in your main base. However this option also advances the time in game.