Hi I think the seedlings have run out of nutrients in the compost. If they have a good root system I would be inclined to plant them out. Dip a hole and pop the whole individual module of seedlings into the hole.
They will grow all together just fine. Maybe give them a boost with diluted seaweed feed.
They only grow to pencil size if you sow direct into the soil and transplant them into deep holes back filled with water. The traditional way.
