For anyone who is interested, apparently jumble has this "feature": if you specify just one single relay instead of the default optimal relays, you will end up with a protected event.
Really wish we could toggle this behaviour, because I can see a lot of instances where it's not the desired behaviour for the user.
I'm simply testing some code tonight against a single relay, and running into this. If you understand the problem, it's easy to just use another client, but it can be confusing if one is unaware.