Not sure if this has been fixed, but I've noticed it in the last few snapshots.

It's EXTREMELY rare, but I've noticed that random blocks of sand or grass seem to be appearing in locations that they weren't before. I suspected it a few times when I saw one grass block in a stronghold, and thought maybe I used it to jump up somewhere, then I noticed a sand block in a farm in a savanna village that definitely wasn't there before. I had convinced myself over the course of several other instances that it must have just been me placing blocks, until I noticed one appeared up against a wall in my base and I know for sure that I didn't place it there, and it was grass, so which I haven't had any in my inventory, only dirt, and there was no nearby grass to convert it to grass if I had accidentally placed a dirt block, and the sand appearing is unusual because I never carry sand around with me. All together, if I had to guess, I'd say it only happened 15-20 times tops that I've noticed, and always a single block. As a note, I am using custom world generation for this world but this is happening in already generated chunks.