Yup, the mesh is to thin for Dynamesh. One trick I’ve developed is to use the inflate brush, and just go over the area on the front and back and really close that hole up. With this technique, you will of course have to dynamesh and resculpt that area.
Also, if I may add, if you are sculpting on a thin mesh, it is really helpful to make sure Backface Masking is enabled on the current brush you are using. This is located in the Brush -> Auto Masking drop-down menu located at the top of ZBrush. It’s a feature I find myself constantly using to the point I’ve made room for it on my UI.