environment variable ORACLE_PDB_SID doesn"t work

Operating system environment is Linux

Connecting To a Pluggable Database as SYSDBA :

The following environment variables ORACLE_SID , ORACLE_PDB_SID are set for "Connecting To a Pluggable Database" , but it is not possible to connect to a Pluggable Database. I heard in an online lecture that an "After logon trigger" is started, is that maybe the problem ? On the database host there are other containers and pluggable databases, where it works without problems to connect.
So you can rule out problems with the OS, here is an excerpt from the connection.

export ORACLE_SID=CDB1 
export ORACLE_PDB_SID=PDB1

sqlplus / as sysdba

SQL>   show con_name;

CON_NAME
------------------------------
CDB$ROOT