Mining-tasks left floating tile that cant seem to be mined anymore

I set the mining-task layer by layer and still they somehow left a tile alone that is now floating and neither rush-command nor direct assignement makes them mine it. Trying to fix it by building a stair up to it, but yea, this shouldnt be a thing :) 

Edit: yea this bugged out, even with stairs they wont mine it, see 2nd screenshot