iCloud no longer backing up automatically

Hi,

Recently I've noticed my iPhone 13 Pro (iOS 15.3.1) is no longer backing up automatically. The iPhone is locked, connected to Wi-Fi and power, and there is enough storage available on iCloud to complete nevertheless, the iPhone is still not backing up automatically.

If I trigger the backup manually via iCloud settings on iPhone then the backup completes successfully within a few minutes, but does not backup automatically as mentioned above.

Any help appreciated.