This has happened to us as well, and we're on 8.1.25.
It doesn't happen by itself, it happens after a project save. The audit log does show that page-config resource and session props were modified. But this is not true, and the engineer certainly did not delete the page configuration, which has a confirmation box on the delete box. We actually had two page configurations, the second was a back-up, and both were cleared. We had to quickly rebuild it.
It can't be replicated.