Sometimes in multiplayer worlds (I was playing splitscreen in case that matters), some changes to blocks on one player's screen are entirely different than the changes on another player's screen. I don't know how to replicate this, but I've seen it twice now. Once (prior to 1.16.100), a friend had changed some of their floor from planks to glowstone, but when I came to see, the glowstone just looked like the normal planks and emitted no light, but when I broke it it had the same mining speed as glowstone and same breaking particles, as well as the same loot table. The second time (after 1.16.100, but I don't think that makes a difference), I had broken some blocks that were in midair because, well, I didn't like them. But when my friend came to check it out, the blocks were still there on their screen. When they started walking on those blocks, they constantly fell through the blocks and then clipped back on top of the blocks, which caused repeated fall damage. I have absolutely no clue what caused this bug, but it should probably be looked into sometime.