Missing sub_packno field value in table ESLL for field srvpos

In our system , I saw missing sub_packno field value in table ESLL for those records which has SRVPOS values.

However, there is sub_packno field value for blank srvpos entries.

Is it SAP standard bug or system behavior, because I am unable to capture sub_packno for srvpos and thus unable to fetch PR against the activity number.

Regards