icloud space

You can try:

See what is using up iCloud storage

1) Open Settings app > General > [Your Account Name] > iCloud (you can manage your iCloud data and see what is using up your storage)

Checking the phone's syncing (if the storage is used by Photos)

This is to see if your phone updates your photos to your iCloud storage

Pathing: Open Settings app > Photos > iCloud photos checked or unchecked

How many photos your iPhone has (if iCloud photos' checkbox is checked)

This is to see if your phone is replacing the now free storage by newer images not yet uploaded on your iCloud

1) Open Settings app > General > iPhone Storage > Photos (this will tell you how much data you have in Photos and therefore tell you if you are overbooking iCloud storage)

2) Open Photos app > Library > Days (at the bottom you see how many pictures aren't uploaded yet in iCloud)

Reloading the page (if iCloud photos' checkbox is unchecked)

1) Quit Settings app > Reopen Settings app > General > [Your Account Name] > iCloud (recheck if your icCloud is still full)

2) Turn off and turn back on your phone, then recheck if the iCloud storage is still full

If this doesn't work please go contact Apple as I am not a professional.

Did you empty the recently deleted album in the photos app on the phone?