iCloud backup data for no device

Hi,

My iCloud storage is affected by a ghost 2.3 GB backup:

Here is what I get by clicking « Manage… » :

But if I click on « Backups », there is no device showing:

Same thing if I use my web browser and manage my iCloud storage, it shows a 2.3 GB for backups of 0 device !

I tried to contact Apple support but they keep asking me to force a restart of my iPhone, disconnect and reconnect my Apple account and now their supervisor is asking me to backup and restore the iPhone using my Mac… how does that make any sense ?

I can find old discussions about that same issue but none of them is giving a solution. Does any of you have any idea on how to get rid of that ghost data?