Users often encounter a "full" error even when no files are visible on the drive. This is usually caused by one of the following:
It is a fake flash drive. the device "goes bad", like not mounting, showing generic names, show weird capacity, etc. Technibble How To Fix A USB Mass Storage Device Problem [Tutorial]
The File Allocation Table (FAT32 or exFAT) has been damaged. Windows reads gibberish and assumes the remaining space is zero.