I just lunched a 1.15.2 world in 1.8.9, because I forgot I wasn't in 1.15.2. Without a warning it opened the world, then I realized my mistake. It was actually a big castle, and now chuncks and new blocks were missing, I was so desperate. Then I randomly found that by going in my world files and rightclicking the file, I had a "restore older version", and when i clicked, i realised my computer made an auto backup of the file the day before. I restaured it, and there I had it, my world with all its missing blocks. (I am still on windows 7)
I just lunched a 1.15.2 world in 1.8.9, because I forgot I wasn't in 1.15.2. Without a warning it opened the world, then I realized my mistake. It was actually a big castle, and now chuncks and new blocks were missing, I was so desperate. Then I randomly found that by going in my world files and rightclicking the file, I had a "restore older version", and when i clicked, i realised my computer made an auto backup of the file the day before. I restaured it, and there I had it, my world with all its missing blocks. (I am still on windows 7)
Hope this can help anyone !