PowerBuilder 2019 R3 build 2728

Windows 10

I am having a silly isue that is probably all my fault, but I can't seem to work it out. I am exporting four datastores to an XML file using this command

   ads.SaveAs(as_file, XML!, true, EncodingUTF16LE!)

and then importing them back into the same datastore at later date using this command

   ll_ret = ads.ImportFile(XML!, as_file)

Three of them work just fine but one of them returns -4 in ll_ret. So I turned on XML import tracing in the datawindow properties and got a log file that has this at the top (Note, I am not using XML templates anywhere)

/*---------------------------------------------------*/
/* 19/05/2022 10:52 */
/*---------------------------------------------------*/
CREATING SAX PARSER.
NO XML IMPORT TEMPLATE SET - STARTING XML DEFAULT IMPORT.
DATAWINDOW ROWSIZE USED FOR IMPORT: 42

and then lists every element in the file with no error messages that I can see. The last item in the list appears to match the list item in the XML.

Anyone have any idea what I'm doing wrong? The file has been exported, not toucjed and then is imported, surely that should be safe to do???

Thanks!