It looks like the order of a lookup table is important when you're passing an array in as the source and expecting an array out of the lookup table. For example, we have a lookup table that takes ad positions in as an array like ["WX_VideoCompanion","WX_VideoPlayer","WX_Spotlight"]. The elements of the lookup table (because of entry order) has "WX_Spotlight" before "WX_VideoPlayer". This causes the lookup to return [,, "nl"]. The last value is the default value if the lookup can't find a value. Even though the value for WX_Spotlight is in the table, the lookup can't find it, because of the order.
The solution is to insure that all values in the lookup table are in alphabetical order and then sort any input array alphabetically. However, as stated before the order in the lookup table is a result of when the entries are made and there seems to be no way to re-order the lookup.
Am I missing something?