The solution to Possible Bug in Logical Standby Auto-Deletion of Logs is to set the
DB_RECOVERY_FILE_DEST_SIZE parameter to a reasonable value.
The source of the problem was that I had set the value of this parameter to a value far greater than the size of the disk used for the FRA. This was done to avoid the hassle of changing the parameter every time a new disk was added to the disk group. We had tightly managed disk space through RMAN for archived REDO log files, so we did not rely on this parameter for disk management.
It looks this value is critical for the management of foreign archived REDO logfiles for logical standby databases. Setting this value to a suitable limit enabled the automatic clean up mechanism to work correctly.