Why does the Disposition automatically change to 'no value' when opening an incident for edit?
Incidents, Workspace, Product Linking
The behavior outlined is standard behavior when product to disposition links are removed.
To correct this, you will either want to re-link the disposition or sub-disposition or update the affected incidents and set a valid disposition association based on their product association.
From the incidents table at the database level, the associated disposition value may still exist. This is why you're likely able to pull reports based on that value. However, once the record is opened for edit, the application displays the product value and adjusts the Disposition field listing accordingly, based on current product to disposition links. Because the previously selected value is no longer linked to the current product selected, it is not displayed when the record is opened. The result of this is that the Disposition field will display No Value.