To replicate:

Create a page with this content:

{excerpt}
This is a test.
{excerpt}

Another line.

When saving the page, the space between the two lines will be removed.

Add a title to the top of the page like so:

h2. This is a title.

{excerpt}
This is a test.
{excerpt}

Another line.

... and the gap will render properly.

Verified in Safari and Firefox.