Extending stairs downwards

Summary: I tried to make a mineshaft and I added stairs after the first I mined the first 4x4x4 block. afterwards I mined another block straight down and I wanted to extend my stairs which the game didn’t allow me to do. I also couldn’t destroy the stairs seeing as they only do that from below so my mining hearthlings got stuck down there.

Versions and Mods: the one before the update from june 3th