Strikethrough (~~) renders as empty angle brackets

I have a situation that I have worked around that seems like it might be a bug, but I acknowledge that this might be by design and that I have just misunderstood something.

I have this markdown in a variable named markdown:

To do:

~~add PDF link to board~~

I have added the emphasis extras and was having an issue. (I have tried both with and without EmphasisExtraOptions.Strikethrough.)

var pipeline = new MarkdownPipelineBuilder()
    .UseEmphasisExtras()
    .Build();
var doc = Markdown.Parse(markdown, pipeline);
var html = doc.ToHtml();

// I learned that it works if I pass the pipeline again to ToHtml in addition to passing to Parse above
// var html = doc.ToHtml(pipeline); 

The following is the generated HTML:

<p>To do:</p>
<p><>add PDF link to board</></p>

I have also tested with ~, +, ^, and all give the same result with empty tags.

I am using:

• version 0.31.0
• ASP.NET 6
• Visual Studio 2022

All that said, if I pass the pipeline again to the ToHtml call, it does render the tags correctly. I would have thought that since I created doc with the pipeline that I wouldn't need to use it again when calling ToHtml.

Is this a bug, or by design? Any suggestions what I am missing?

Start with the Markdown.Parse and doc.ToHtml calls shown in the issue, comparing how the pipeline is retained and used by each operation. Reproduce the sample with UseEmphasisExtras and verify the expected strikethrough HTML; done means the parsed document renders correctly without passing the pipeline again to ToHtml.