I suppose the issue is caused by the excel will record the last used cell when saving the file. When opening the file again, it will automatically redirect to that cell. So that, it will automatically navigate to the middle of the excel file when opening it using Excel web part because the last used cell is in the middle.
You can follow the steps below to reproduce that:
- Create a excel file and enter some data in it.
- Click in a cell, for example, cell R5, and then save the file.
- Upload the file to a SharePoint library.
- Open the file from the library, you will find it will automatically active cell R5 and it will be scrolled to the R5 to make it available in the screen.
- Render this file using the Excel web part, the same situation. It will automatically active cell R5 and it will be scrolled to the R5 to make it available in the screen.
- Edit the file and click in the cell D5, then save the file. The last used cell is changed to D5. When opening the file again, the cell D5 will be active.
You can locate the cell at the beginning of the worksheet section when saving. Upload this file and use this file in Excel web part, the issue should be resolved.