for the data to be stored and retrieved properly, the flash drive must be working properly. Consider the following diagram, and be in awe of the graphic skillz I possess...
possible reasons:
- some electronics failed on the PCB of the flash drive, now it doesn't work properly.
- bad connections / solder joints
- firmware of controller chip corrupt, now it doesn't work properly
- NAND Flash is faulty, now it doesn't work properly.
- The data got corrupted due to a bug in the firmware
- cheap flash / device fail
etc..
a Data recovery company might be able to unsolder chip, read it and recover files. Or try taking a disk image with DMDE, ignoring partitions and run data recovery software such as getdataback, and choose options like "ignore File Allocation Table", find lost files, "raw scan" etc