First Footer of a section overrides the First Footer of another section after the merge. After merging 'Parent' document (with a different First Footer which is empty) with 'Child' document (with explicitly set First Footer), the First Footer of 'Child' overrides the First Footer of 'Parent' after PDF export.

Workaround 1: Disable 'Child' document's 'HasDifferentFirstPageHeaderFooter' and clear its First Footer's value:

Telerik.Windows.Documents.Flow.Model.Section section = child.Sections.First();
section.HasDifferentFirstPageHeaderFooter = false;
section.Footers.First.Blocks.Clear();

parent.Sections.First().Footers.Add(HeaderFooterType.First);

Nested Mail Merge cannot handle empty lists.

Workaround: Replace the empty list with null.

Merge field group in a single table cell causes it to replicate more times than expected.

Workaround: add a second column to the table and move the TableEnd/GroupEnd/EndGroup/RangeEnd merge field inside it.

Import/Export of specific documents sets style background color to black.

Workaround: do the following conversion of the document with the Document Processing libraries: originalRtf -> exportedDocx, exportedDocx -> finalRtf. After that processing, the finalRTF file will no longer reproduce the issue when exported.