There are two tables in all arenas. Objects and Arena. Object holds downtimes and hardness, and Arena holds the crafts at the right fields, and environment variables like sunrise, sunset, radar range, visibility, etc. As admins, we have NO ability to save Object tables - only Arena. If a map resets, we have to re-do the object table, but we can reload the arena in a snap.
When we get a map reset - like we did on Wednesday, the object table sometimes doesn't load right. We, as admins, don't know this is an issue, and will re-load the arena table which sets all the fields up right and we carry on. Players join, destroy stuff, then everyone leaves at the end of the day - the arena is now empty. The next person that comes into the arena the next morning, could cause the map to reset.
Why? We don't know, but we have a work around. We'll ensure this is done from now on if there are any future map resets. Expect one, for example, next Monday, for end of the month stats.