Thanks for the template file and output HTML files.
After an initial test, I am able to reproduce the issue as you mentioned. I found that drawing shapes are not rendered fine in the output HTML file format.
e.g Sample code:
Workbook workbook = new Workbook("e:\\test2\\renderDrawing.xlsx");
workbook.Save("e:\\test2\\out1.html", SaveFormat.Html);
I have logged a ticket with an id “CELLSNET-46937” for your issue. We will look into it soon.
Once we have an update on it, we will let you know.