The preview shows non-bold text for H2 level headings like ## Overview as shown here:

I don't think this stands out well. I thought about changing it, but I decided it wasn't worth the effort. However, I let it slide, and lo and behold it rendered in bold, just like I wanted it to!

If the rendering engine has been improved from WYSIWYG to semantic syntax to telepathic inference, then let me congratulate the devs on this accomplishment.
Otherwise, it's a bug in the otherwise perfect Javascript rendering tool.