Why does my NPC stop moving mid-walk to a point?

There is a time limit to MoveTo. If it does not reach its destination point it will fire MoveToFinished with false.

You could use this and run the MoveTo function again.

local humanoid = script.Parent.Parent.Giant.Humanoid
local vecList = {
    -- list of positions vector3
}
local curPoint = 1 -- holds the current walk to point

humanoid:MoveTo(vecList [curPoint]) -- the first point 

humanoid.MoveToFinished:Connect(function(foundEnd)
    if foundEnd then
        curPoint = curPoint  + 1 -- go to next point if found
    end
    if vecList [curPoint] then -- check we have a vector3 to walk to
        humanoid:MoveTo(vecList [curPoint])
    else
        print('end') -- no more points
    end
end)

I hope this helps.