Multiple base paths that share a common string cause double replacement

I have a 'Plain HTML' custom text format whose only filter is Pathologic. I've globally configured it thusly:

Processed URL format = Full URL

All base paths for this site:

• http://www.livesite.org
• http://stage.devsite.com/site
• http://stage.devsite.com/site_d7

The first item is the live website. The second is the Drupal 6 version of the site which we've copied content from, and the last is the new Drupal 7 version of the site with the copied content (body text which includes links, etc) from the Drupal 6 version.

This is the HTML I entered for an image with the 'Plain HTML' filter:

<img src="sites/default/files/styles/thumbnail/my-image.jpg" />

This is what I expected when viewing on the http://stage.devsite.com/site_d7 domain:

<img src="http://stage.devsite.com/site_d7/sites/default/files/styles/thumbnail/my-image.jpg" />

However, this was the result:

<img src="http://stage.devsite.com/site_d7/_d7/sites/default/files/styles/thumbnail/my-image.jpg" />

As you can see, there's an extra, errant '/_d7/' path argument in there.

When I changed the base path list to the following:

• http://www.livesite.org
• http://stage.devsite.com/site
• http://stage.devsite.com/sd7

I had absolutely no problem at all and things rendered by Pathologic worked as expected:

<img src="http://stage.devsite.com/sd7/sites/default/files/styles/thumbnail/my-image.jpg" />

I'm certain this occurs when two base paths share the same string if matching from the beginning of the string.