This was discovered in the new test (fcbis_clear_appliedThrough_after_recovery.js) I added with "Extend Backup Cursor" for FCBIS as part of SERVER-62745. From my discussion with Matthew: We get the size storer from the initial snapshot, then we disable size adjustment during oplog replay. But for oplog entries added after the initial snapshot's recovery optime, we probably DO want size adjustment. I'm surprised we haven't seen it, we might be missing test coverage for extend.