That would indicate that the file file has been damaged somehow, or there is a bug in the compression library. The latter seems very unlikely. Since the compression phase is entirely deterministic, it would mean that certain combinations of input data would always produce compressed data that would cause this problem. The library is used by us and by so many others, it’s really unlikely (though of course nothing is impossible). My guess is the file has been damaged externally somehow, after the fact.
What we’d need to do is to verify the structure and checksum of the file. We don’t actually have this function, because we’ve never really needed it! So, we’ll have to implement this first before knowing exactly what has happened. See https://bitbucket.org/axantum/axcrypt-net/issues/297/add-feature-to-verify-file .