I’m not sure how your mesh is constructed, but is it possible the hair piece is an open or 2d volume? Dynamesh doesnt work well with geometry that isn’t a closed volume with a degree of thickness. The usual cause of holes like this when dynameshing is geometry that is too thin, or 2 dimensional.
This goes for most things in Zbrush as well. Most functions in zbrush work best on closed volumes with thickness.
If you have access to a version of the hair before it was dynameshed, you can use Mesh Extracts or ZModeler to extrude a 3d version of the hair. OR you can make sure the hair is completely intersected with the head mesh with no open gaps, and dynamesh will attempt to close the volume and delete the interior overlapping geometry.
Mind, the hair and the head need to be part of the same subtool to dynamesh them together.
Repairing the mesh at this point will be ugly. You are basically going to have to use a clip brush to flatten the problem area close to the skull so that no holes are remaining, make sure there are no gaps between the head and the hair, and hope Dynamesh can fix it then. You will have to re-sculpt the hair back into shape.