1) What is the problem?
When changing owner of a database, the owners of all tables belonging to this database are removed.
2) For example:
2.1) user_A is owner of DB1, user_B is owner of DB1.TABLE1, and user_D is owner of DB1.TABLE2.
2.2) admin with" ALL with grant option" changes the owner of DB1 from user_A to user_C through command "ALTER DATABASE DB1 SET OWNER USER user_C".
2.3) Now, user_C is owner of DB1, there is no owner on DB1.TABLE1, and no owner on DB1.TABLE2.. In other words, the owners of DB1.TABLE1 and DB1.TABLE2 are removed when changing owner of their database.