Is there any apps that are requiring that 1gb+ remaining space to be able to back up?
Maybe because you only have a little over 1GB left, and iCloud starts to constantly bug you storage is full. I encountered this once when the storage was less than 2GB left for 50GB plan.
If you have iCloud backup turned on then you’d see the message when you don’t have enough space left to perform a full device backup.
If you have iCloud backup turned on then you’d see the message when you don’t have enough space left to perform a full device backup.