I've found the solution for this "error" and it is none, let me explain: I've been dealing with this bug for almost a year and I've tried everything and when I was giving up I've realised that this isn't an error, I barely check my saves folder in .minecraft and when I did I've realised that all of the worlds I've downloaded from my realms is in my saves folder, it just doesn't show up in-game and when I changed the names of the save files they appeared in-game and everything was fine so my hypothesis is that minecraft thinks there is an error in the extracting stage but in reality there isn't an error it just thinks there is. Well I don't know if this works the same for other people but I hope this helps you.
MC-211309
I've found the solution for this "error" and it is none, let me explain: I've been dealing with this bug for almost a year and I've tried everything and when I was giving up I've realised that this isn't an error, I barely check my saves folder in .minecraft and when I did I've realised that all of the worlds I've downloaded from my realms is in my saves folder, it just doesn't show up in-game and when I changed the names of the save files they appeared in-game and everything was fine so my hypothesis is that minecraft thinks there is an error in the extracting stage but in reality there isn't an error it just thinks there is. Well I don't know if this works the same for other people but I hope this helps you.