The heading wikitext cancels out the bold wikitext.
I went to the control panel and looked through the palette editor but couldn’t find anything to edit to keep the H1 and H2 and H3 (etc) buttons from interfering with the bold formatting.
It is a problem with font-weight that it fixed in the prerelease, the fix
the heading elements have a font-weight of 300 and the element has font-weight bolder.
The bolder of 300 is 400 (=normal)
You can see the specification about this property here