For example, in Volume 2 during the Goblin arc, there was a girl the MC met in a village, who was later revealed to be a noble who escaped after her family was killed. She played an important role as her presence allowed the MC to piece together the mystery of the goblins, and afterwards the goblin problem was solved, she is never mentioned again. The whole reason she traveled along with the MC was to find her brother who enlisted to fight against the goblins, she thinks he is alive because she gets sent flowers from her brother every so often, and it is later revealed that her brother paid a flowershop to periodically send flowers to her, and is most likely dead. Did she ever find out the truth? And what happened to her after? Well guess we'll never know.

In Volume 4, Fallen Angels who sought to revive their god they follow were the main antagonists of the volume, the MC fought them in the past and the present, and stopped the revival of their god both times. In the present, the MC called for reinforcements from the Sky City, which made them retreat, those guys were the strongest enemies that the MC has ever faced before, and even in later arcs, they are still considered strong, so you would think that they would make a future appearance, but nope, they are never mentioned again afterwards. Well, the MC at the very end of the novel would probably 1 shot them if they ever appeared again, so...