It looks like Bethesda mixed up the workshop vs non-workshop versions of Shack Stair 01. The non-workshop version (formID 00075804) has the required stairhelper (white plane visible in CK collision view) which makes it easier for NPCs to use stairs placed in workshop mode. The workshop version (formID 000E0BB1) does not have the stair helper. Simply adding object navmesh and swapping the constructible object to the non-workshop version should fix this problem.