Hi,
Let me explain the issue based on the sample document attached :
TestDoc.docx (17,8 Ko)
My sample document has a TOC relying on TC fields (\f switch) and using Chapter prefix (\s switch). The chapters are based on the SEQ fields existing in the document. These SEQ fields are formatted with the switch: * ALPHABETIC
When rebuilding the TOC (.UpdatePageLayout()/.UpdateFields()), Aspose does not apply the ALPHABETIC Format (here S,T) and set the TOC entries prefix as numeric (here 19, 20).