Sometimes when you try to place sand/gravel in place that will cause it to fall with the new gravity mechanics it will disappear and not be placed or fall. The sand/gravel will also be used out of your inventory.
Also if you remove a block of sand/gravel at the bottom of a column of sand/gravel the whole column of sand/gravel above it disappears as well.
If sand/gravel falls on sand/gravel it will disappear, if it falls on other blocks like dirt it won't disappear.
Also if it falls on snow it will float there. Attached picture of this.
Ignore comments about only disappear on sand/gravel tried this again and its disappearing on most blocks now.