Disappearing Default Template

For the second time now, the default blog template I created has disappeared from the Blog Templates list. This happened before and I cannot pinpoint any activity for why the setting went missing. Figured it was a fluke, but it just happened again.

I noticed that a new blog was created, using the default 2011 theme instead of the theme and widgets I set up in the site that I set as a default template.

Any idea why the default template would disappear from the list and no longer be applied? We've added no new plugins, and can't see any other reason why the setting no longer gets applied, all of a sudden. After I recreate the template, it does apply to new sites. But this is now the second time I've had to do this.

Any feedback is greatly appreciated.