I first ran into this issue three weeks ago right after A16’s first dev build. At the time, I chalked it up to save compatibility from A15 to A16. Since that time, I started a fresh A16 game circa ~D2940 and just continued playing it again with D2955.
Promoted my trapper to a shepherd, defined some livestock zones and watched his AI in the job management UI. When he stayed idle for too long and started hauling, I unchecked Haul and he just stayed idle and only tended to his own survival needs. Demoted back to Trapper and back to Shepherd, no luck. Eventually gave up and re-demoted him back to Trapper.
Version Number and Mods in use:
A16 D2955 x64