So working with about 10 small Pivot Tables in Calc 7.0.3.1, one per sheet, and in saving to .XLSX file format found that when I re-opened the file all of the Pivot Tables were converted to a simple text table. Took my starting point .xlsx file and saved as .ods and then recreated the Pivot Tables and they save and work correctly. So something not working correctly in File - Save As .xlsx. Fails the same way whether starting with the file as an .xlsx file and adding Pivot Tables or starting as an .ods file and adding Pivot Tables then saving as a .xlsx file.
May be one of the plenty Pivot Table bugs related to xlsx format listed in this META bug tdf#112416 - (Pivot-Table-XLSX) - [META] XLSX pivot table bugs (Don’t use foreign formats, if you don’t have a good reason for doing so).