When all type block on the falling sand entity falling on the cobweb before disappear (MCPE-12765), then break the cobweb the first falling entity will replacing the other when the block was placed. As shown in the picture the falling sand: red sand, sand, gravel, and anvil falling on the cobweb at once, then when the falling sand touch the ground, then break the cobweb the other entity like anvil, and the red sand will replaced by the regular sand (the regular sand I put that first before the other).