In the dress' default shape, that middle part was part of the stretch. The BreastFlat morph doesn't flatten it out properly. It seems the morph was created with an automated tool, such as "copy morphs" in Poser. Those are hit and miss and sometimes details like this ought to be cleaned up afterward.

dlfurman posted at 11:49PM Tue, 10 December 2019 - #4372991

Is that an area to simulate the stretching of cloth across the breasts?