I know this is a common issue but I haven’t used Zbrush is years and I’m not sure of a work around. I had to delete a blending edge on a part and I had been using Dynamesh to turn it back into a solid mesh but I need to fill in this small gap along the edge. Dynamesh tries to fill in the whole back side of the mesh. You can see what’s going on in the attached images. I hope there’s some way to limit Dynamesh but noithing I tired works.
I’m pretty sure dynamesh can’t identify that gap and bridge it, but you could try cutting that gouge back into your dynamesh result with the trim curve brush. Alternately you could split to subtools, dynamesh both, inflate the convex one a bit, and either dynamesh subtract or live boolean subtract, then clean up a bit.
I imagine there are better solutions, hopefully someone has them on hand 
Perhaps manually bridging a few faces across the gap with zmodeler or curve bridge will give dynamesh or close holes a better idea of where it should focus.
What I do is turn on my “Draw Polyframe” and use Zmodeler to create a couple bridges from one shell to the other. Then close holes.
Here it is.