First, that isn’t a bunker.
It looks a little bit to me, that is a kind of “foundation” for some type of shelter.
Maybe for a wall tent. “Bushcraft” is kind of a hobby with a lot of people these days. Looks like more trouble than it was worth, whatever the purpose. But for winter camping, it could be useful, some sort of home made deal, wood stove, etc.
The inclusion of sandbags and the size is a bit odd
Otherwise I’d say a hunting ground blind.
That’s still my guess.
Great rain collector