Campaign Collectable no longer achievable.

I was replying the last mission, Always Forward, trying to get all the collectables. For the second to last one, which is before you get the stinger missile for personal use and before the tank section, there is a collectable there. I walked too far forward without noticing and when I tried back tracking, I was out of bounds already. I could still see the collectable so I ran over with a knife to make sure I could reach it in time. However, it simply just vanished and didn't count. I replayed the mission to get to the same spot again but the dog tag wasn't there and I couldn't complete it, leaving it as my last collectable.

I'm sure this can be replicated with other dog tags, but they'll need to be attainable within the time limit while out of bounds.