This can happen when running a BACKUP SEQ=PHYSICAL with the default of UPDATE=NO and MULTUSE=NO.
This form of the backup will run in single user mode. If the MUF is active and the area is open for update when the backup runs it can miss records because it does not pick up the records that are still in the MUF pipeline buffers.
In addition, if compressed records are being updated at the same time as the backup is taking place, records could move and be missed by the backup.
To prevent the problem run a DBUTLTY LOCK OPTION=MOVER before the BACKUP SEQ=PHYSICAL to flush the buffers to disk and prevent the movement of compressed records. After the backup run an UNLOCK OPTION=MOVER. For example:
LOCK OPTION=MOVER,DBID=nnn,AREA=xxx BACKUP AREA=xxx,DBID=nnn,DDNAME=ddname,SEQ=PHYSICAL,RECID=NO,CMPRS=YES UNLOCK OPTION=MOVER,DBID=nnn,AREA=xxx
Alternatively use one of the following methods:
Specify SEQ=PHYSICAL,UPDATE=YES to ensure the area is not opened for update.
Run in multi user mode by specifying the parameter MULTUSE=YES in the BACKUP.
Do a BACKUP SEQ=NATIVE which will not have this problem because it prevents the database from being updated during the backup.