Quest NPC disappearing

NPCs regularly disappear after giving out quests, rendering these impossible to complete.

Specific currently ongoing issues:

• Major disappearing after I have reached lvl20 marksmanship skill, I can't complete the quest for the sword.
  I currently have 2 Hectors, one at the craft table, one at the exit beam of the Playa.

• Hazel disappeared - I have the Cake quest finished, but it still appears in the quest log as active quest.

• Daily Quest NPCs are gone as well etc.