Hi Andrew,
Both of the PDB and RDB are on 15.7.
the entire new system is migrated from earlier 15.5 version.
File system type on old 15.5 server was of type UFS whereas the newly created 15.7(where log truncation issue) are uses ZFS file system.
I am not sure if this is related to file system issue or something else.
Will keep you posted if I could able to get correct solution and root cause.
Regards,
Swapnil