Ideas
I have this exact issue as well. I missed it at first, ran back into the gray zone and picked it up, didnt pop up, i died and now it's gone.
Here is where it should be.
And here is the tracker showing that i haven't collected it.
Basically locked out of completing the campaign, i can't move forward in the mission until my goblin brain is has gotten the shiny dog tag.
So I have checked on internet and this particular dog tag is number #2 in the last mission, and not the #3.
I have not found the last one and I mistook the #2nd collectible for #3rd and #3rd for #4th, when I have not seen the last one yet
So you have the same situation, there are still two collectibles to be found by you
Sorry to create confusion, but you are good still two more collectibles in the mission for you to find.