When writing a question a few minutes ago, I logged into Data Explorer in a private window. When I logged back into it in a normal window, my username had changed to "Stormblessed31729" due to not being unique. Even after logging out of the private window, it was still allegedly not unique. Logging back in in the private window, it was still Stormblessed there.
Changing it to something else there then changing it back in the normal window fixed it.
This is a weird bug and likely a symptom of another problem. I'd like it to be fixed at some point, please.