what means "other npcs" in your sentence?
do you mean that any merchant (that you defined) can be spawned as long as it does not list mantle, or do you success to spawn all NPCs (type 2 to 13) but merchants (type 1) ?
if you success to spawn merchants, does it mean that all merchant types but the Mantle-Merchant one (subtype 7) can be spawned ?
if so, please confirm that the issue occurs even with empty merchant (npc with no listed items).
if finally all merchant types are spawned but the mantle one, an unexpected check is possibly performed by server (to forbid these NPC outside of capitals - yeah why boring the svr with such stupid test?...); if that's true you have to insert the mantles in the list of any other merchants (not a mantle one); if you edit your NPCQuest with shStudio, I (unfortunately) define the types of available items according to the selected type of merchant - whereas the purpose of the category is only to define the generic description displayed under the NPC name; all merchant types can obviously sell all types of items.
If it offers a way to solve that issue (and possibly others similar) I can (at least) allow generic-merchants (subtype: 0) to sell any items (of not only the types 25, 44 & 100).
thank to let's know about additional tests you may perform.