I am having the same issue but on my PS4
Idk how to downgrade to 1.12.1 but I would if I knew how cause I really want that world back
this same thing happened to my world on my computer and my friends and I had spent a long time but it got marked as a duplicate though the one they think is my duplicate is for a realm
This happened in a normal world not a realm
I figured out how to fix it, you just have to turn your anti-aliasing down to one and it will all go back to normal.
The exact thing happened to me right after the update I really want the world back because me and many of my friends worked for months and had so much stuff and, like you, I want my world back if there is anyway to retrive it.
My world is 150.0MB, that is the biggest world I have had on my ps4 and am scared it will corrupt. Also I don't know my version, but it's one of the two I listed.