I just tried running Minecraft with the latest Forge and nothing else but vanilla 1.12.1, and it still happened. Running vanilla without Forge, I still have my normal name. Whether or not there's anything in Forge that should change my name, it appears to be what's doing it.
Whenever I use Forge lately (including but not limited to the latest version), my in-game name changes to "Red_Master". This also causes my worlds to not recognize that I have command privileges. I can't find any cases of this happening to other people. https://pastebin.com/GpUzsje3