Disclaimer: I know POI well, but nothing about its handling of pivot tables.
My first question would be: why use such an old version of POI? Since 3.14, at least 3 changes/enhancements have been made to pivot table handling according to https://poi.apache.org/changes.html
, described as "Pivot table enhancements", "Support Table (structured reference) sources in PivotTables" and "XSSFPivotTable::addColumnLabel sets the cell type of a cell outside of the source data area", respectively. While it's not clear that those apply to your situation, it might be worth the effort to try the latest released version.