On a 4k monitor...
If you create a from-scratch winforms app, and drop in an HtmlLabel, the control will render correctly in the designer, but it will be sized 1/4 as large as it needs to be at runtime. Since the HTML rendering itself is in correct proportion, this means most of it is outside the control and is truncated.
If DpiAware is added to the manifest, the autosize feature works correctly again.