• There are two ways that Questie applies nameplate icons.
• 1: When nameplates come into view, via NAME_PLATE_UNIT_ADDED which calls QuestieNameplate.NameplateCreated.
• 2: When the quest log progress changes, via _EventHandler:QuestLogUpdate() which calls QuestieNameplate:UpdateNameplate() whenever the user adds/removes/progresses a quest.

The properly working situation is as follows:

• User is far away from quest mobs.
• User accepts a quest. The UpdateNameplate code runs. There's no work to do, since the user isn't near that mob.
• User gets into range of quest mobs, the NAME_PLATE_UNIT_ADDED handler runs, adds the mobs to the activeGUIDs table, and draws nameplates with the proper Questie icon. That handler also runs frame:Show() on those icons to ensure that they are visible.

The bug situation is as follows:

• User is standing next to quest mobs that have zero icons. Because there's no icons, the activeGUIDs table doesn't contain any data for those mobs.
• User accepts a quest which involves the nearby mobs.
• Because the mobs are already visible in range, their nameplates are already visible, so the NAME_PLATE_UNIT_ADDED handler never runs.
• The only handler that runs is UpdateNameplate, which then loops over the contents of activeGUIDs. But that table hasn't registered the mobs when their nameplates came into view, since the player had no quests for those mobs. So Questie doesn't react to the fact that those mobs now need icons. It is blind about the existence of those nameplates. In fact, even if there had been data in activeGUIDs, it would still fail to show icons for them, because UpdateNameplate only sets the icon texture, it never calls :Show() for the icon.

To sum up the two bugs:

1. Nameplates are ONLY registered in Questie's activeGUIDs if the user has quests for those mobs when the nameplates come into view. If not, then Questie is blind about the existence of those nameplates.
2. UpdateNameplate doesn't call :Show() for the icon, so it would still not appear even if activeGUIDs had tracked the nearby nameplates properly.

The only real question is: Is it even worth fixing these bugs, and making Questie track all nearby nameplates at all times, and properly apply quest icons if quests are gained that relate to nearby mobs? Is it a common enough situation to need a fix? Maybe not.

Small sidenote, which should be fixed at the same time:

QuestieNameplate:UpdateNameplate() does the following useless double-check (I've removed all surrounding code):

if icon then
    if (not frame.lastIcon) or icon ~= frame.lastIcon then
        frame.lastIcon = icon
        frame.Icon:SetTexture(nil)
        frame.Icon:SetTexture(icon)
    end
end

This is pointless because first it checks if icon which means icon isn't nil if that check success. Then it checks if (not frame.lastIcon) which means "if lastIcon is nil". Lastly it checks if icon ~= frame.lastIcon which means "icon" and "lastIcon" are different.

This should be rewritten as:

if icon then
    if frame.lastIcon ~= icon then -- Reacts if lastIcon is nil (since icon is non-nil), or in any other way differs from icon.
        frame.lastIcon = icon
        --frame.Icon:SetTexture(nil) -- This line should be removed, it's literally pointless. May have been some ancient WoW bug.
        frame.Icon:SetTexture(icon)
    end
end

I noticed this myself, but didn't have the time to look into it yet. It is also quite easy to fix from a user perspective by simply toggling the nameplates once.

So I agree it might not even be worth the time, but it is a bug and I will leave this open!

Still happens