When you import a solution that contains an entity into Dynamics CRM, which has a relationship with activities through the regarding attribute, the cascading settings for the relationship will revert to whatever the value is in the solution.
This occurs even if the solution being imported does not make any changes to these relationships or the cascading settings. Additionally, if the requirement level or label of the regarding field for any out-of-the-box activity is changed, and a solution containing an entity that has a relationship to activities is imported or an update is applied, the changes will be reverted. This is because update rollups or updates to CRM Online contain solutions that are imported.
After you import any solution, apply update rollups for CRM on premise, or apply updates for CRM Online. Make sure that you confirm that any modifications to the cascading settings, labels, or requirement levels of the regarding attribute for activities are configured correctly.