I think it happened a while ago and just noticed that ALL of my characters, both eternal and seasonal have names that are different than when they were created. Since you cannot change the names yourself, I have no idea how they were changed.
Sounds like you have enabled Streamer Mode, which changes any and all names that appear on screen.
To see what the hotkey is to enable/disable it, go to Options > Controls > Scroll about halfway down to the System Section > Toggle Streamer Mode.
I don’t know the default key off the top of my head as I usually change about 95% of all keybinds in all my games.
All of my character names have been changed without my authorization. I have disabled streamer mode.