After getting the 3rd character up to level 46, I think I've found the problem(?).

There are indeed level 46 quests. However, there's only one Lv.46 quest line accessible to our first class. This is due to the story making the other Lv.46~47 Waking Sand quest givers invisible. The only accessible one is the Odin line that brings us to Quarry Mill, where we are able to get another two or three associated side quests.

Once we get through the main quest, we will be able to see the various Lv.46 quest givers in Waking Sands. Hence there are those who say "lazy noob L2P and look for them", while in fact the story progression blocks us out of the quest givers.