Symptoms

ID column in K2 generated form changed from Auto Number to Number after regenerating the SmartObject.
 

Diagnoses

OOTB the generated SmartObjects will use Number as the type, we do allow you to make changes to this, in this case change to AutoNumber. When regenerating the Smartobjects K2 this will be reset to Number as this is what SharePoint provides K2 as the type for the ID column.
Changes are supported but k2 will not preserve it when regeneration is done of the SmartObject and this is by Design.

The same will apply for other customization on the OOTB generated SmartObjects
 

Resolution

After regenerating the SmartObject all customization will be reset and they will have to be added back manually.