Can't change undo tablespace location in PDB RAC 12.2

Hi, after change to LOCAL UNDO MODE in Oracle 12.2 I can't change the default location of UNDO tablespace.

I can specify only one undo tablespace but after doing that, the two instances try to use the same datafile after a reboot.

( ORA-30013: undo tablespace 'UNDOTBS1' is currently in use )

I have tried "With Local Undo enabled, How To Set Specific Undo Tablespace For Each PDB Instance On RAC Node (Doc ID 2673826.1)"

But it seems like it is only for 19c because it is no taking effect in 12.2c

Where is the spfile for PDB'S? I can't see any information about how the PDB select the undo tablespace, is like that it is getting a default one for each instance: