I found a way to fix this issue on windows, not sure if it works on other devices
Follow this file path C:\Users\\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftpe
Once you're there, open the file "options.txt" in notepad, or anything that lets you edit it. at the top, where it says "mp_username", the name that keeps popping up instead of your actual username will be there, so just change it to match your username, save options.txt, relaunch minecraft, and it should work. Hope this helps!
Cr0wEternal
I found a way to fix this issue on windows, not sure if it works on other devices
Follow this file path C:\Users\\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftpe
Once you're there, open the file "options.txt" in notepad, or anything that lets you edit it. at the top, where it says "mp_username", the name that keeps popping up instead of your actual username will be there, so just change it to match your username, save options.txt, relaunch minecraft, and it should work. Hope this helps!