I have the same problem. A couple of days ago I bought 50GB icloud space and I made a 18GB backup by my Iphone7plus. Well, I should admitt that I'm new about icloud, but I always used backup on external servers using my PC windows and in 22 years of work I never lost a single byte. When I saw the total amount of 18 GB used space I was sure backup was done. So I started the update of my iphone and it stopped to work for a day, I was ont able use it as phone also (my old Motorola phone saved me!!!)...only black screen with that "bitten apple". So I went to the reseller for a techincal help. He said to me that the only chance was to format it, losing datas, but I was sure to have done a backup. So they did a total format of it and when I fill all my datas, it started to work properly. The icloud told me a 18 GB backup, so yesterday night I left him close to my router and with the power connected and started the restore of backup for the whole night. This morning the bad evidence: the icloud utilized for backup space was 4,3 GB....I find on my iphone only few datas, not a picture, not a movie, not whatsapp....
Where my 18GB datas was gone?
I feel myself very angry!!!