Spiral borer inconsistency

On the 19th or 20th layer of a spiral borer borehole, there will be a break in the sets of downstairs. However if you take an alternate staircase down and return to the borehole, it continues down. There is even an up-stair that takes you to the location where the "missing" downstair belongs.

It seems like the borehole should either: * terminate after 20 levels or * continue to dig uninterrupted to infinityyyy