A previously working package call returns ORA-04067

One of our jobs that executes hourly started erroring with ORA-04067. No changes to this package were made before this error started. The package body became invalid, but compiled without issue. After restarted the database, the package call was successful. Any ideas on what the root cause was? The following statement produced the trace below:

SQL>  exec notification.run_hourly_alerts();

BEGIN notification.run_hourly_alerts(); END;

*

ERROR at line 1:

ORA-04067: not executed, package body "APP.NOTIFICATION" does not exist

ORA-06508: PL/SQL: could not find program unit being called:

"APP.NOTIFICATION"

ORA-06512: at line 1

Trace File

Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production