Custom theme lost, config entry blanked

Hi folks,

I logged into my cloud this morning and noticed all custom theming had vanished.
When I opened the config file, the theme setting was blank: 'theme' => ' '

I’ve put the theme name back in and all is now normal, but I’m curious to know how this would have happened?

NC 10.0.1

Seems to be intentional whenever an upgrade is performed.