I’ve tried assigning only one worker per field, yet I always see two working on the field.
It is the same whether I auto-refill professions, or assign manually.
The crops do rot still if only one worker is assigned, but I still see more workers in the fields than are assigned.
The crops are working as they should, but laborers / farm workers assigned seems to be wrong.
This is not a bug. Farming is a communal task in Frontier, and farmers work together across all fields to complete their tasks. Assigning farmers to individual fields does control the total number of farmers assigned in your settlement, but it is not an absolute cap on how many villagers will work that field at any given time.