That's how it starts and mine, to fill up more and more, with apple garbage, because it is impossible that recently restored, and almost without use, it shot in an alarming way. and this has been happening since iOS 14, but instead of filling up in an uncontrolled way, it directly corrupted the partition and always showed 0Kb. In ios 15, filling System Data has happened to me almost since beta 1, few builds have been able to contain this. The worst thing is that before you restored and this was solved for a while, but now it is not, which indicates that something must be wrong there.