wpGallery class instead of wp-gallery

Working on parallel on two sites running WP 3.6-beta3, I’m bumping into a very weird issue:

On one of the two sites, Image Galleries are not displayed in the visual editor. They are just fine in the text mode.

Inspecting the HTML, I see that the generated markup looks like this:
<img class="wpGallery mceItem" data-mce-src="https://example.com/wp-includes/js/tinymce/plugins/wpgallery/img/t.gif" src="https://example.com/wp-includes/js/tinymce/plugins/wpgallery/img/t.gif" alt="" title="gallery ids="100,101,102"">

This microscopic image does not get styled, so the gallery remains invisible in the editor.

On the other site, the markup looks like this:
<img class="wp-gallery mceItem" data-mce-src="https://example.org/wp-includes/js/tinymce/plugins/wpgallery/img/t.gif" src="https://example.org/wp-includes/js/tinymce/plugins/wpgallery/img/t.gif" alt="" title="gallery ids="100,101,102"">

In the second case, the CSS class is wp-gallery instead of wpGallery, and it’s getting properly styled.

On the non-working style, I disabled all plugins, and reverted to the TwentyThirty theme : issue is still persisting.

Pretty crazy, isn’t it?