Can't unlock Codex Entry in "Over The Top" (pc)

I have a small game breaking bug in this mission that prevents me from unlocking all the codex entries and thus preventing me from getting the achievement for it. In the campaign called "Through Mud and Blood" you're supposed to destroy all the field guns in "Over The Top" to unlock a codex entry. The problem is that all the field guns won't spawn in my game. I've watched numerous walkthrough videos and replayed the mission from the start but there are always one or two field guns missing from the places where they should be.

To make the issue even more confusing, the location of the missing field guns is randomized every time. So basically when I find a missing field gun and restart the mission from the beginning, the missing field gun then spawns where it should be but then some other field gun has disappeared in its place. It's just impossible to unlock the missing codex entry and achievement because of this issue.