@quentinmsl Version 1.20.2.r6 also enable flash wear leveling. Before that change, LFS would always use Block 0 and 1 for the superblock. If you write really often to the file system, like millions of times, these blocks get damaged. But if you still can reformat the file system and start over without any files, than the flash should still be fine.
But if the first two blocks are damaged, the only cure would be to create a firmware, which places the file system at a slightly higher address, leaving the damaged area beside. Or you replace the flash chip, which is some work.
When you reformat the drive, does the file write problem start immediately? Or is it a special file name/situation which causes it to fail?
Error 8 indeed stands for LFS_ERR_CORRUPT, telling that the file system is defective.