Excel 07 issue opening MHT format
 

My code generates MHT content that we named as .xls file so it can be opened
in Excel. Some of the worksheets are set to be Hidden, which seems to making
some problem with Excel 07. The steps are following:

1) Open the file in Excel 07
2) Click "Save" in File menu
3) Close the file, reopen it again, there is no hidden sheet any more

This is a problem because I used the hidden sheet to hold the data port of a
chart. After the above steps, the chart is showing empty data.

This doesn't happen with Excel 2003, and if the worksheet is not hidden, we
don't have a problem.

Is this some bug of Excel 07?