This is indeed a bug. However, you can easily fix this by adding a custom style and a custom clip:
1. Select the main stylesheet and add the following code:
Code:
span.strike {
text-decoration: line-through;
}
(If you haven't created a stylesheet yet, create a new one and link it to the .html files)
2. Press CTRL+ALT+C (or select Clip Editor... from from the Tools menu)
a) Click Add Entry and paste or enter
Strike into the Name field.
b) Paste
<span class="strike">\1</span> into the Text field. (You might have to double-click the Text field to edit it.)
c) Click Save and Close.
If you've folĺowed these instructions correctly, you should be able to apply your new custom Strike style to selected text by right-clicking the selected text in Code View mode and selecting Clips > Strike from the context menu.
If the span was correctly inserted in Code View mode, but you don't see strikethrough text in Book View mode, you'll need to link the stylesheet to the .html file via the Link Stylesheets context menu in the Book Browser.