Board Game Arena

In the same round (solo game) I first placed a worker on "farmland" and plowed a field. Then I placed a second worker on "cultivation", expecting to plow another field and then sow both. However I was not offered the plowing part, only the sowing action on the first field.

Is this a bug or a rule that I overlooked?

Most likely you didn't have a legal place to plow another field. All plowed fields (except the first) must be adjacent to an existing field, just like fenced spaces and rooms.

Oops! From memory, I think you are right. There might have been a pasture in between.
Thanks!