Do you move the website backup role from one server to another?
My backup server had problems after v12upgrade, so i created a fresh new ext4 backup server and it worked correctly, BUT when i moved the websites backups from the one temp server to the new one, every movement multiplied the usage. I moved 200GB from a server to the fresh one and became 800GB (and the new one was working incremental and fine!).
I had just to wait the retention policy, and than after a week, thanks to rotation, i have again 200GB used. Don't know if this is your case, but if you moved backups, than i think that you encounter my same issue 🙂
