In my save file, all builder slots are available with about half of them “Idling-unable to work” and not automatically going back into the labor pool.
Could you please upload that save?
Saves are located in:
C:\Users\YOUR USERNAME\Documents\my games\Farthest Frontier\Save
Zip up the entire TownName_###### folder along with the .map file and .sav that reproduces the issue. If the files are too big, you can use a service such as Google Drive to share it with us.
Thanks!