Cows de-spawning

I gathered 2 cows and breed them till I got about ten. Logged out for 7 hours to sleep and in morning when I returned they were gone, Server auto reboots at midnight to help with the lag I had been getting. I rolled back to the last backup which was 4am EST and server had already rebooted at midnight. The cows returned from the restore. but they should never had despawned