Why can't I change my world's name?

I've never had an issue with files before in mine craft but I think ever since the last update I can't change my world's name anymore since usually when I make a copy of world it will say "Copy of..." I'll make a few saves and delete the "copy of" since it's annoying to read and I won't get it mixed up in the other copies or something.

Anyway, now when I try to edit my world's name it won't let me change it anymore. I made another world just to see if I could change that world's names and it's copies and I was able to for those save but not my initial world. Has anyone else had this issue?