There is currently an issue in Minecraft regarding the crosshair (split controls) with the latest update that rolled out, crouching in the game has been changed, making it similar to Java editions.

The current problem is that when you crouch the place where your crosshair is pointing does not move down, the best way I can explain this is that when I crouch my crosshair stays in the same place on my screen and looks normal but in game it is pointing in the place it would be if I were standing up so if I try breaking the bottom block of two blocks stacked it will break this top block

this is extremely tedious, especially with slabs. please see to it that it gets fixed as it makes building very difficult.