I'm putting this here, in case it helps anyone out there who's chasing that "Hostile Takeover" achievement like myself. I can confirm that disabling Settlement mods such as those which alter the building area, height and/or settler population solves the problem, at least for the non-quest related landmarks such as red rocket and coastal cottage, essentially for settlements taken over by creatures which you'd clear out yourself to be able to unlock the workshop. :happy: