If you just open your excel file in Microsoft Excel and save it as HTML, you will get the same results as you get with Aspose.Cells. So it is not a bug.
It seems, this feature is supported by Aspose.Cells but not supported by MS-Excel. We have logged this issue as CELLSJAVA-42088. Once, there is some fix or other news for you, we will let you know asap.
This is to inform you that we have resolved your issue logged earlier as “CELLSJAVA-42088”. We will soon provide you the fix after performing QA and incorporating other enhancements and fixes.