steps to reproduce:
when i place a stackable block (e.g. rails), and quickly use a hotkey to switch to a different slot (i used keybind 3 for rails and keybind 4 for tnt minecarts), it switches back to the first item (rails) without placing the second (tnt minecarts). this is present with other blocks such as obsidian and end crystals, or stone and cobblestone.
expected result: rail is placed, switches to tnt minecart, places tnt minecart
actual result: rail is placed, switches to tnt minecart, tnt minecart not placed, switches back to rail in a very short amount of time
i think this happens when switch happens on same tick as placing, but i am not sure.
a similar issue is seen in mcpe-189526, but not exactly the same