Yes, this is a known bug since one of the last releases (perhaps the last) ... It is in some CSS header styles that are meant to set spacious margins, without affecting top header, but it is applying incorrectly for other headers also. I am looking to include this in a maintenance fix, but in the meantime, you can set inline style for the first header.
The bug is in fact with the "first" header, not the successive headers. You can try to add 3-4 headers, and you will see they are the same, except for the first one. It may seem like a lot of margin, but it is generally quite comfortable spacing between paragraphs in a scrolling environment.
I temporary fixed it on mjau-mjau.com, by setting first h2 header manually:
<h2 style="margin-bottom: 0.8em;">Single Page Applications</h2>
See it here, with default spacings, which I think are ok: