Had the same problem. It was solved though I am enraged and thoroughly dissatisfied with their reasons for why this happens.
Whenever you see an excessive amount of storage that's counted toward "Docs" and nothing on your device actually accounts for those documents, you need to call Apple and ask for a senior adviser.
The following gibberish response was what I got from them:
"Apple reserves a certain amount of space to ensure you are not using all of your storage. When you see your icloud storage full, you have to call us and ask us to 'release' this space"
The senior adviser I spoke to (will not name) gave me this **** and bull rubbish, nonsense reason for why I had 23.6 GB, yes GIGAbytes of my 50 GB upgraded storage withheld from me. My husband has the same upgraded storage plan and his icloud functions exactly like it should. His "Docs" (withheld storage - if that is even real) amounted to a measly 1GB with 37+ GB of 50GB used. Go figure!
I've had to deal with 'Icloud full' messages for the last 3 weeks. I'd keep deleting something hoping the space will eventually be freed until I actually went and looked at the actual use of the space.
So yeah - Solution is go call Apple, Ask for a senior adviser. They need to manually "release" this reserved/withheld/whatever the eff storage for you.