When the miners were digging from the surface, they were very logical and dug with a mind toward getting out of the hole again. Once they dug below that point, they became stupid.
They dig all the dirt out of a section except for a narrow shelf they refuse to touch, even with a ladder to get them up to it.
They dig down first, and then when they become trapped, they refuse to dig outward until they can get back to the hole from the top again.
The solution to problem 1 seems to be to make sure there are enough ladders leading to all the ledges and then keep at your villagers until they finally remove the ledges.
The solution to problem 2 is that there needs to be a ladder that leads both to the bottom of the shaft as well as another ladder to the top of the part you’re excavating. This enables your miners to get to both the bottom and the top of the excavated area.