WP removing title attribute for someone else but not me?

We’re using WordPress 3.1 and have two network users, me (who’s a network admin) and a colleague who’s just a standard network user who has access to a blog that we’re both editing.

When I save an <img> tag in a post with a title=”” attribute everything is fine, however when my colleague does the exact same thing it strips the title attribute from the <img> tag but leaves everything else intact (including any other changes they may have made to the tag). In fact, if they load a page with an <img> tag with a title attribute but don’t change anything, on saving the post (it’s currently a draft) it will forceably strip out the title attribute, which it won’t do when I repeat the exact same process.

I’ve only been using Firefox 3.6.13 on Mac OS X Snow Leopard 10.6.6, they’re using the same OS but can reproduce the problem in several browsers (including the same version of Firefox).