It is because you have impromptu set as a write relay.
Your notes are being served in the home feeds of your followers who are using outbox-capable clients, like Amethyst. Those clients send replies & likes to your read relays and all the read & write relays of the people who interact with you. They don't create protected replies and the original notes are being duplicated on any of those relays that ignores the "protected" tag.
Impromptu will only work as intended if you go directly to it to post.
wss://lockbox.relays.land will work as I described above, too, but on purpose. That might be a better one to try. It doesn't accept any replies, so you don't need to add it as a "read".