Changing Schema ownership and side effects
In SQL Server, changing the schema owner will drop all permissions. This is a "feature" buried deep in the documentation:


  If the target entity is not a database and the entity is being
  transferred to a new owner, all permissions on the target will be
  dropped.


https://docs.microsoft.com/en-us/sql/t-sql/statements/alter-authorization-transact-sql

For a schema, this apparently applies to the permissions on all objects within it.