While I was trying to figure out how to make a mortar barrage inaccurate enough to not be able to metagame (remember how the mortar shells fell and mess with it) I came across Placement Radius in the transformation tab. It was the answer.

I tested it in the editor, and it worked wonderfully. I saved the mission, uploaded it to a server.

I tried it in the server, and every single mortar shell hit the exact same spot.

I went back to the editor, noted the placement radius had dropped to zero. I fixed that, saved, and reuploaded the mission.

Every single mortar shell hit the exact same spot. Again.

Which means it's probably a save issue.