FI Substitution - Z Fields

Hi,

We are facing a problem regarding FI Substitution. We have defined some additional z-fields in table BSEG through Append structure. Now we have also defined substitutions for those fields. Now when we are posting one FI document through FB60 that substitution is working fine and the custom fields in BSEG are also getting populated correctly. But when we are doing Internal Order settlement to another Internal Order through KO88 then the substitution is getting triggered as in the previous case (FB60). We have also debugged the substitution and the z-fields are getting substituted within the debugging. But when the document gets posted and we see the details of the document in table BSEG, those z-fields are blank. The substitution is defined at callup point '2' (line item). Kindly suggest.