Custompress field values disappearing

It seems that if I change anything about the custom fields, eg assigning them to more post types, the values completely disappear from existing posts. I've had this several times now after making various changes, which makes me very nervous.

I'm using the Thesis theme, which otherwise works perfectly with it. I notice there's a similar post a year ago (https://premium.wpmudev.org/forums/topic/custom-post-custom-field-data-disappears).