Mongorestore w/o oplog replay rebuild all the indexes, but Mongorestore with oplog replay doesn't rebuild indexes

MongoDB version: v5.0.20
Community version
Mongorestore - works fine and all index rebuild successfully after restore
But when run Mongorestore with oplog replay, restore and oplog replay completed successfully, but no indexes were being rebuild after the restore.
Why? Mongorestore will not rebuild the index if run with oplog replay option ?
Please help !

Thanks !
Sally