At first I suspected it might be a purposeful change that was done, ie. "Oh, people wouldn't really want to go back to their Home Page!" Like when they removed the option for the new tab page to open the home page (though they did that for security reasons). Now you have to install an add-on like new-tab-override to set the new tab URL.

But since it doesn't happen every time, it's more likely a bug. It may even be related that particular add-on, since it's the last one I installed.

Yes, once I figure out exactly what is causing the problem, and if it happens on a new install too, I'll report it.