Old thread.
My experience. The sheath itself doesn’t dull the knife, but working in dusty, gritty environment more than once it’s gotten sand, and grit in it that has dulled/chipped the edge.
Run a big mower, weedeat, climbing under houses etc, and if knife is exposed pull it out to use it, stuff falls down just waiting for the knife to be re-sheathed, and once it’s wedged washing won’t get it out, it takes either wire to dislodge it, or back blowing with compressed air. Some of it is how Sheath is designed also.