As I noted before, the mission marker did not appear during patrol after Wet Work (which follows Radio Silcence). I only got it to appear after Crossing Paths.

Update: I tried getting to this area during a Patrol mission after Wet Work (I do not have a Patrol save for after Radio Silence). No Abandoned Battlefield marker appeared on approach. I was able to rush through the forbidden area on an ATV, and no mortar fire came in after I got to the base.

I will see if I can do the same during Patrol after Crossing Paths.

Update 2: I was able to rush through the mortar barrage on an ATV. However, this would still make the mission unreachable on foot.

Similar thing happens during Episode 2. After the latest update, all FIA units, including Kerry, are spawned with random head- and eyegear instead of those assigned previously.