When sculpting a 3D model with blend shapes, breaking often stems from conflicting vertex positions between the base mesh and blend shape targets.
Blend shapes rely on identical vertex counts and order—sculpting that alters vertices (like subdividing, merging, or not disabling blend shapes first) creates mismatches, causing distortions.
To fix or prevent this: Disable blend shapes before sculpting to keep vertex data consistent. For existing issues, re-align vertex positions between the base and targets to resolve distortions.