Save FrameMaker 2020 to 2019 via ExtendScript bug

When I save a FrameMaker 2020 file down to FrameMaker 2019 using ExtendScript, the top line of the file still says <MakerFile 16.0> so it won't open in 2019. It works in the interface with File > Save As.

#target framemaker

var doc;

doc = app.ActiveDoc;
saveDoc (doc);
doc.Close (true);

function saveDoc (doc) {	
    
    var params, returnParams, i;
    
    params = GetSaveDefaultParams ();
    returnParams = new PropVals ();

    i = GetPropIndex (params, Constants.FS_FileType);
    params[i].propVal.ival = Constants.FV_SaveFmtBinary150;
    i = GetPropIndex (params, Constants.FS_AutoBackupOnSave);
    params[i].propVal.ival = Constants.FV_SaveNoAutoBackup;
    i = GetPropIndex (params, Constants.FS_FileIsInUse);
    params[i].propVal.ival = Constants.FV_ResetLockAndContinue;

	FA_errno = 0;
    doc.Save (doc.Name, params, returnParams);
    if (FA_errno !== 0) {
		PrintSaveStatus (returnParams);
	}
}