Using Redgate Schema Compare 184.108.40.2066 command line to rebuild database schemas from RedGate generated script files.
We rebuild multiple schemas in one pass - so have an automated run that builds each schema in turn, but logging on each time as the SYSTEM user (this worked fine in 220.127.116.112)
The schema build scripts generated by Schema Compare for Oracle 18.104.22.1686 OMIT THE SCHEMA PREFIX FROM PACKAGE NAMES (in create package and create package body commands) - So when rebuilding the schemas, it attempts to create stored code within the current schema (i.e. SYSTEM)!!
So after the schema rebuild, we're left with multiple broken schemas that need to be patched up manually.