I have the same problem, I have a iPhone XR, with 64gb of less than one year old. The other storage is 37Gb big, the memory is completely full, can’t even open my mail. I have offloaded almost all apps. It seemed that with offloading the problem got worse… One month ago I had the same. A reset reduced other storage to 5gb, but I am not looking forward to a reset every month. Yesterday, the other storage was 36gb, today already added another 1gb. It is really unacceptable. Apple support only suggest a factory reset. My old iPhone SE with 16gb is even more useful than this phone!!