ok so we wanted to mine all of that area and the villagers did, except for two blocks, so we made a ladder up on one side and they mined one block and left, leaving the other block to rest in the air. this is one case out of many that we’re experiencing, some are where the villager’s mining path leads them to being trapped or not completing the job.
hmm… in order for them to finish i think you’ll need to add a new ladder on the block, as i don’t think they can reach it if it’s floating in the air like that. 
yea but having to go back and add ladders to a block constantly is a pain when there could possibly be a change in the hearthling mining priorities, such as mining from out to in, instead of mining in to out. so that both blocks would be mined instead of just the closest block.
the difficulty in implementing that is that the hearthlings mine from their “feet” location, so they have to be standing on or in front of the block they want to mine. They can go 2 blocks above their feet level, but only if they’re standing directly against that block they want to mine.
To fix this situation, the hearthlings would need to figure out whether mining a given block would make another block within the order inaccessible… for a small area that wouldn’t be too bad, but for a large area that would be an insane resource drain on your CPU/RAM.
I think that a simpler solution would be to have the hearthlings simply build ladders on a block-by-block basis whenever you give them an order to work on something they can’t reach. They already do it with building, so it would be intuitive for players that they’d reverse the process rather than players having to manually build those ladders.
Of course, I have no idea how difficult it would be to make the game figure out when a ladder is required for mining; but I would guess that it’s easier to go that way than to have the hearthlings “pre-simulate” the mining task for every block to figure out if they can accomplish it.
I seem to get this issue only when I am mining a 5 block column. Normally the mining tool only highlight a four block column but you can make it do 5. when you do this the Hearthling will mine row 2 and row 3 and then use that to step up and mine row 4 and 5. However if the Hearthling gets hungry or tired, and does not finish a block in row 5, another Hearthling may come along and mine the block in row 1 so now there is now no way to step up to get row 5 without a ladder since this leave an inaccessible block.
edit: That is why in cases like this I mine from the top down
