While we don’t currently have published SLAs, it is a goal of this feature for customers to be able to create their own custom dashboards on this managed instance, and for those dashboards to persist. Data is stored on a Fly Volume so durability expectations are consistent with that feature.
That said, while this service is still in preview things are being changed around quickly, and we’re still working through some application-level bugs/issues that can impact custom settings or dashboard data. On Aug 27-28, a bug in the dashboard-provisioning logic caused all existing dashboards to get unexpectedly deleted, and we didn’t recover this lost data from the daily backups in time. Apologies to you and any other early-adopters who lost work as a result. Making sure something like that doesn’t happen again is a top priority for this feature.