ORA-12850 running ash_wait_chains against 19c database

TFA is reporting "CHA Problem Cause : CHA detected that commits were blocked for several seconds because the Log Writer Process (LGWR) was blocked."

I thought I would use Tanel's ash_wait_chains.sql to dig in a little more but:

SQL> _at_ash_wait_chains event2 1=1 sysdate-1 sysdate Error report -
ORA-12850: Could not allocate slaves on all specified instances: 3 needed, 1 allocated

I did some googling and some searching on support.oracle.com to no avail. I've got an SR open but was wondering if anyone else has hit this and/or has a solution to prevent the ORA-12850.