I am of the believe that the current description of the stacker, in the codex, can be easily misinterpreted into this(link), in fact I will go as far as to say that it does not state that shapez support horizontally and the “With empty or loose quarters parts may fall further before landing” implies that the right part of that white shape should fall as it a “part” or a shape above “empty quarters”. Idk, someone native to english reading this feel free to call me an idiot if im completly wrong.
https://discord.com/channels/1000343719314198548/1524902813455355925