I did some additional testing and confirmed that this is purely a visual issue caused by the browser not rendering the falling cells quickly enough.
For example, in my screenshot below, the bug makes it look like Mistral is standing on top of a quartz sand cell, but the quartz has already been dug up (my quartz count increased by 1). When the player moves afterward, the cell image refreshes, causing the quartz to appear as though it suddenly disappears.
