After having tons of times where Hearthlings dig strait down and then are stuck until someone builds a ladder - I had a thought.
What if Hearthlings could build a ‘Rope Ladder’ which can be placed on the ground anywhere, and if the terrain is dug up next to it, the ladder automatically unrolls itself down. That way its still up to the player to place these ladders (hearthlings wouldn’t do it automatically), but they would be able to place the ladder before mining rather than afterwards - removing the risk that hearthlings become stuck.
By default these ladders should be a height of 4 to be the same length as a default mining cube, but there could be additional lengths in case you want to dig waaaay down in a strait line. Would probably be easier to have it so you could drop a rope ladder onto another rope ladder to increase the length it drops down, that way you don’t need multiple lengths.