Question on Quest NPC and loot tables

[Thunck]

Gee whiz question, but are quest NPC that are spawned by a quest item, supposed to have a % chance to drop the subsequent quest item or is it supposed to be 100% guaranteed?

For example. Captain Berradin in the Coldain Ring quest chain/series. If you attack him without respawning him by giving him the Avalanche Ale he has a normal dwarf loot table. If you give him the Avalanche Ale he is supposed to respawn (new ID) and when killed drops his head and a letter to Peffin (both items are quest items). Those items are needed to continue the chain to Peffin, then to Doldigun, then to Juliah. Each subsequent NPC needs the quest item from the previous to trigger a respawn (to their quest version/loot drops).

TLDR: Is a quest spawned NPC supposed to 100% drop the quest item?