I had the EXACT same problem. I dont know what the heck is up with it. I was trying to hide an objective through the init.sqs, and in SP it showed it at the Breifing Screen. But if you hit continue button, and go to your map, it will be hidden then. LOL, messed up! Must be a small bug. 1.92 may fix it??? Hope so....
-=Kilo=-
P.S. If anyone has a mission with a WORKING* Hidden objective through the init.sqs file, it would be appreciated if you could send that to us.