Even though you export the solution as managed and then import it into the same organization, all the changes, which have made to the organization do not revert back.
1. Create a publisher and a solution.
2. Open the solution and do customizations, like adding Entities, adding Fields, removing values from OptionSet.
3. Export the solution as managed, delete the unmanaged solution and install the managed solution to the same organization.
4. Now uninstall the solution, expecting all the customizations have been removed to make the organization in a default state.
All the modification still remains. The objects you created as still in the organization and values which has been removed has not revert back.
Even though you created a new solution on top of the organization, all the customizations, which has been made in the solution are applied to the Default Solution. Therefore, even though you export the solution as managed, all the customization remains.
This is because a new solution is considered as an unmanaged solution, and it is part of the organization. This behavior is By Design.
No workaround. When you develop your solution, please customize the organization for that purpose only.