Hi Toby,
I have no experience of this happening. I also tried searching for an instance where such issue occurred to one of our customers but couldn’t find one.
Rather than a WordPress bug, one potential cause for this might be a server side issue or how the site was pushed? I also tried searching for an instance like this occurring in Google be also couldn’t find one.
We also don’t have any code or functionality in our products that could cause such issue.
Moreover, another cause that I could think of is if a backup file which has the parent GeneratePress theme activated was accidentally restored. Then, effectively overwriting the current file which has the child them activated.
Maybe you could try reaching out to your hosting provider and see if they have any inputs with regards to this?
Hope you get to the bottom of this as soon as possible. 🙂