Also having this problem as of tonight. I was playing last night and had no problems, so this bug has been introduced in the last 24 hours. Same as everyone else, my hearthlings start building and only complete about 20% of the building before giving up.
For anyone else experiencing this issue, a handy workaround is to open the console (ctrl+c), select the building and then type “ib” (without quotes) into the command line. Hit enter and the building will complete. For crafter buildings (like blacksmith, etc) you will need to save and reload your game, otherwise your hearthlings can’t use the craft stations for some bugish reason.
Devs, loving the progress you’re making. Fixing this bug would also make the game infinitely more enjoyable for me! (I currently can’t place furniture or wall fixtures in or on existing buildings, meaning hearthlings can’t have nice things!)