I see that this is a ZSphere preview. Did you store a morph target while the ZSpheres were visible and before you started sculpting the preview?
Every time you turn on the preview, ZBrush calculates an ideal mesh based on the current ZSphere positions. During a single session that calculation normally comes out the same due to memory caching. But if you save your work and start a new session the cache is lost and the calculation can turn out different. This then messes up any sculpting or texturing that you’ve done. But if you store a morph target while the ZSpheres are visible, ZBrush calculates a mesh that is then saved as part of the ZTL file and always used from session to session – unless you add or remove ZSpheres.
In short, when you’re sure that the ZSpheres are the way you want them store a morph target. Then turn on the preview to begin your sculpting or texturing.