As you know, when you create a mission it stores everything in full file/folder format for editing until you save the file as a single/multiplayer .pbo. (Usually in C:\Users\UserName\Documents\Arma 3 - Other Profiles\Profile for the full file/folder, and C:\Program Files (x86)\Steam\SteamApps\common\Arma 3\MPMissions for the .pbo)

When you open up your mission list, you will see two copies of the same version. The .pbo version will show up in white text, while the editable/full folder & file version will show up in blue.

If you click on the blue version and hit Edit you can make edits to the mission files like nromal.

The problem is, after you've made the edits, if you click on the "Preview" button in the editor, it actually launches the pbo version (white text) and not the full folder/file version (blue text) that you just made edits to.