Well first of all, someone else knowing your username should not be able to lock you out of your account. So even if it isn't a bug it's bad design.

It looks like you can still sign in even if your account is "locked" which further adds to the theory that it is a bug.

Here are a few threads with a bunch of "me too"s. [0][1]

[0] https://www.epicgames.com/fortnite/forums/battle-royale/roya... [1] https://www.epicgames.com/fortnite/forums/battle-royale/roya...