tinyMCE text editor removes numbers, strips content

I’m encountering a problem with custom HTML in my new WordPress site. When I add my preformatted HTML, content with numbers is stripped out.

For instance, if I input this code into the tinyMCE editor and hit “Update”:

<p>3D photography was a popular hobby in the 1950's</p>

…The editor updates with this truncated version, which is also what gets published to the site:

<p>a popular hobby in the 1950's</p>

My HTML tags are preserved, it’s only the content that gets truncated. In my profile settings, the visual editor is disabled (I’m a front-end dev and disabled it right away).

I don’t have many plugins installed, but I did deactivate one that I thought might be causing problems: Preserved HTML Editor Markup v1.5. However that didn’t fix the problem. My other active plugins are Facebook, jQuery UI, and WP Simple PayPal Cart.

I’m using the Gridiculous Pro theme, if that makes a difference. (I don’t think it would, but what do I know?)

This bug wasn’t bothering me before, but I did recently upgrade to 3.5.1. Maybe I didn’t notice it right after the upgrade because I was preoccupied with CSS chores.

Anyone seen this behaviour? I’m not finding anything on Google. Thanks for your time!