Why are pdb$seed objects listed when e.g. querying cdb_tablespaces?

Oracle SE 19.19, Multitenant withup to 3 PDBs

In most of our databases pdb$seed objects are not listed in the result set when e.g. querying cdb_tablespaces. However, in some of our databases they show up:

select con_id, tablespace_name from cdb_tablespaces;

1   SYSTEM

1   SYSAUX

1   UNDOTBS1

1   TEMP

1   USERS

2   SYSTEM

2   SYSAUX

2   UNDOTBS1

2   TEMP

3   SYSTEM

3   SYSAUX

3   UNDOTBS1

3   TEMP

3   USERS

Is there any explanation why this happens? What do we need to do to go back to the behaviour that pdb$seed objects are not showig up.

Thanks for any help

Andrea