Whatsapp local backup
Most likely your phone is a bit aggressive about killing the background tasks. So, WhatsApp is unable to wake up the phone at its usual 1-2AM slot to perform a backup. A lot of phones are doing this to ensure a better battery backup and standby.

Do note that notifications may still flow even when WhatsApp is not allowed to do tasks related to backup in the background. But most of the times, they are either delayed or only show up when you first turn on the phone display in the morning.