Lime kindly made my diamond mythic sword netherite but after turning my mythic sword to an elytra and later back to a regular mythic sword it went back to a diamond sword. Happened on mn but due to the fact that I am too broke to afford smithmaster I can not reproduce but I will once someone can help me. Also while you are at it I still don't have the fancy forum tag for owning the mythic sword!
Thanks to TankNoodle I got this video This was on main, no more than 5 minutes earlier he upgraded it for me on main north
It's not meant to keep it's netherite state. If you turn it into something, and back into the Mythic sword it will always revert to it's diamond state. You can keep changing it to netherite if you like, but you'll just lose that change once you swap it. This is intended and will not change. PS: The mythic upgrade is being kept only for those who bought the sword this season. More information will be available on this when I update the 2.0 news post.