@sukesh.kotian,
Thanks for providing information. We can observe the issue where Reports List.xlsx file does not show data in MS Power Query, however if we open same file in Excel and save it back on disc, the data is visible in MS Power Query.
It is also suggested that you should not use LightCells APIs to generate such a report containing Pivot table or using Power query. Please generate the report in normal mode and do not use LightCells APIs and give us feedback. LightCells APIs is used for simple reading or write huge list of data, other objects or element of spreadsheet might be discarded when using LightCells APIs.
We need your updated sample application(runnable) for creating this file here as per above suggestion and identifying the issue. Please send us simple console application for our analysis.