Modelled outside of zbrush or modelled with topologytool?
If its any other software, like maya, id try cleanup.
If its made with topology tool it could be a tiny mistake you made, I have learnt to look carefully for those. When you welded together the last horizontal split (right in the middle of the problem area) it is possible that the snap in zbrush fooled you and didnt merge with the vertical edge, but actually created a new loose vertice.
Its just a thought how it could have happened, because if that is the case, I have no idea how to fix it anyway:( except going back to the retopo model and delete and remake that area.
(if theres a duplicate co-planar face you could actually try to mask it and polygroup it with “visibility group”, the splitfeature in the subtools would be able to make that broken face a subtool of its own)