gpg --edit-key
setpref

Set symmetric cipher, hash, and compression preferences.

This sets the preferences in the public half of the key, i.e.,
~/.gnupg/pubring.gpg but does not set them in the private half of the key
secring.gpg. This can be seen with gpg --list-packets -v ~/.gnupg/secring.gpg

The problem then occurs when --export-secret-key is done. This exports from the
secret keyring, including the not-updated preferences.