That's weird. Use the <del>text</del> tag instead: text
I'm sure you are using the fancy (and broken) text editor for making posts that has buttons for making text bold, italic, etc (also known as the WYSIWYG editor), so I don't know if it will work for you or not, I don't use that editor.
If you are wondering how to enable/disable the WYSIWYG editor, there is a checkbox for it in the profile settings page: https://powdertoy.co.uk/Profile.html