Well, sometimes cleanup is evil lol :bandit:
I usually go through these problems with CDPO display.
Custom polygonal display, border edges and face normals displayed.
Then with some xray, i start gathering infos on double vertices round border edges (xray to be sure about selections). If problem persists, i look at the normals.
Then if there's no more double vertices nor any false normal. Then there's a non manifold from which I often get rid deleting the two adjacent polys. Directly appending them back.
Also, before beginning this clean up, select ALL vertices of your mesh, then delete. I think i do this naturally every 5 minutes lol.
Hope it helped