These items "bounce" due to the fact that they are on the part of the cauldron that is 1 meter high if you want it to go in the cauldron then you must get the item toward the center more the same effect occurs on player entities.
This isnt a bug _ you just have silk touch I on the pick which is why when you break the grass you get grass blocks this is the same for stone if you break stone in a cave it usually drops cobblestone but with a silktouch pickaxe it drops stone instead of cobblestone.
Best Master
These items "bounce" due to the fact that they are on the part of the cauldron that is 1 meter high if you want it to go in the cauldron then you must get the item toward the center more the same effect occurs on player entities.