There should not be 256 zero bytes at the end of all files. Something is quite broken. I can’t think of any misconfiguration or bug that would cause such a thing, though.
Are you sure it’s all piece files?
Even the oldest ones, and even the newest ones? Piece files for different satellites?
What about other files on the same volume, or in the same directory if any? Do they have the mysterious zero byte endings as well?
Finally, can you try to run the is-valid-sj1-blob utility on the pieces? If you have Go installed, it should work as go run storj.io/storj/cmd/tools/is-valid-sj1-blob@latest <piecefile>. Or if you don’t have Go installed and don’t want to install it, we can send you an executable if you let us know what platform and architecture to build it for.
You might also try running is-valid-sj1-blob with the -find-length option. If your piece files are valid but have extra bytes on the end, this will determine that.