Hi Liow,
Thank you for the details.
I am afraid that I am not able to reproduce the problem.
I marked A1:E7 as not protected except C5. I opened the document using XlsIO, modified data at A1 and saved. The generated document preserved the protection correctly. I have placed the sample in the link below:
http://files.syncfusion.com/support/samples/XlsIO.Web/7.3.0.20/F89345/main.htmCould you please try running the sample and send me the modified one so that I could recreate and find the cause of the same?
Regards,
Geetha