Definition lists lack preceding vertical whitespace

When definition lists are rendered, they lack the preceding whitespace of other types of lists, and even paragraphs. This makes them look "crammed" into the text above, particularly with headings. I believe the relevant documentation page for definition lists is here. Examples:

h2 heading

This is some text of a normal paragraph, definition list follows.

def 1
: definition of def1

def 2
: etc

here is another h2 heading

• this is how a ul list is rendered

and another h2 heading

def 1
: this is how a defn list is rendered. that 'def 1' is practically hidden up there!

The behaviour that concerns me is in a readme file that is being rendered, but the whitespace issue seems to occur in this issue tracker as well.