It's there. I just checked on PC EU. It's a little difficult to see the yellow sparkles because it's in a patch of bright sunlight, but it's in the room with the delve boss, in the center of the room, in a grassy patch with bright sunlight shining down, near a few pieces of Dwarven trash and a centurian (not the boss). In fact, if you approach the room from the eastern entrance and stand in the corridor before entering the room, the leg is visible in the grass toward your left, without anything else around it, the way you can see treasure chests and nodes in the distance without rocks or grass or other objects around them because you aren't close enough yet for those things to be drawn-- well, depending on your graphics settings in the game, I guess.

Oh, well. I'm on PC, and it's not sunken into the ground there, but I can't speak for any of the consoles. Are you sure you didn't already collect it? I mean, I suppose you know whether or not the construct has a leg attached, but is it on the construct's left or on your left?

You were right. I missed the right leg on this character. I should have checked the achievement first.

No way to tell something is already taken on the ground. Doh!