Welcome to the forums. There are similar reports in Oneplus forum and, unfortunately, there isn't a clear fix. It seems to be caused by a hardware issue (modem) that makes the system create dump files that are never deleted. There's a chance that these files could be deleted with root access, but I haven't seen any user reporting success.
The problem seems to affect different models and most reports come from OP 9 and Nord series. This is just one example.
https://forums.oneplus.com/threads/t...issue.1518111/
I'd recommend you to contact OnePlus support and see what they say.