minecraft 1.12.2
minecolonie 8.6037

2 farmers, set to manual assignment.

I destroyed every scarecrow. Then saved and quit. Then reloaded. Then went into the gui. It showed no fields. So I created new scarecrow and placed them. it still showed no fields assigned when right click on scarecrow. A OK .

Then went to farmer 2 and click on "Barley" field to manually field. Went to field, it showed farmer 2? A OK
Then went to farmer 1 and went to the assignment gui. EVERY FIELD showed up and was able to assign the field to farmer 1 also. So I can assign "Barley" field to farmer 1 even though it is already assigned to farmer 2.

The gui should reflect that fields as disabled like if you have it in auto mode. You should not be able to assign a field to another farmer when it is already assigned.