I had the same thing happen to me last night around in the same location of the Airfield at the tents. Unfortunately I don't remember how much of each ammo I had in the box, but it was a mix of .45 and 5.56 ammo.

I also walked around the tents to make sure it didn't re-spawn somewhere nearby but never found it again.

Happened to me twice yesterday and once (so far) today. I was running every single time it occurred. I will try some the suggestions in the notes and will stop every few minutes to drink or eat something.