Editing an attribute (not the geometry of a feature) of a versioned feature class with a true curve or multipatch causes the geometry's properties that represent the true curve or multipatch to be lost.
If the geometry is a true curve, the properties of the curve will be lost and the feature will be generalized as a line segment.
If the geometry is a multipatch, the properties of the multipatch will be lost and the feature will no longer display.
The cause of the problem is when performing a versioned update, the values of the row being updated are inserted into the versioned feature class' adds table. For versioned updates, the geodatabase must discover all values of the object that are being updated (the remaining attributes that are not being updated by the user). Obtaining these values is accomplished by querying the lineage of the version being edited to discover the last representation of the object and fetching the rows values.
In the case of true curves and multipatches, the complete geometry is not being obtained and the insert of the row being updated into the adds table loses these properties.
This issue has been addressed in the latest general patch for ArcSDE 9.2 Service Pack 6. See the Related Information section below for the ArcSDE 9.2 Service Pack 6 Memory Allocation Patch.