Under “Development Tools > Statistics” you can end up with lines with “Fix Issue” where the only option is to remove the statistics. This happened to a lot of my entities recently when an upgrade changed all of the ones ending in “_battery_level” to just “_battery”. I think it also happens if you rename an entity_id.
It would be nice if you had the option to merge the old statistics into a current entity with the same device class, rather than losing the data.