When using the command of /fill for a comparative large range (even 50*50) in minecraft realms, it causes a bug that when breaking the blocks or placing new blocks in the selected range(in that /fill command), the game gets stuck and, updates very slow and sometimes causes a "no respond" and even a game crash. It affects EVERY members in the realms who are online. This problem can't be fixed by just restart the game. The only way to fix the problem now is to download the world, reset the old one and re-upload.
Note that these situation WOULD NOT cause this problem:1.filling blocks vertically 2.if the block broken is not the lowest block of the horizontal platform
Another issue is that our world is a flat world with lava on the bottom part. I don't know if this will happen or not in normal world.