This is tremendously aggravating. I find that this blog mysteriously changes back to the default Wordpress theme without warning at completely random times. After turning on lots of logging, I found that the GoogleBot and regular visitors seemed to be causing it, but they caused it when they made completely normal requests from the site.
After getting flustered, I realized a dirty hack was in order. I made a new theme folder called ‘default-original’ and moved the contents of the default theme into it. After that, I copied the contents of my desired theme folder into the default folder. After waiting a few hours, the magic happened and the theme was reset. But alas - the default theme is now my desired theme.
Dirty hacks sometimes work the best. 
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[...] Wow, so you spend all that time coming up with a custom template that matches the web design of your main site, and lo and behold, the wordpress theme changes back to default! Does anyone out there have a fix for this misnomer that doesn’t involve doing this? [...]