Indexes and Dictionaries aren't really much use: they aren't stored by any useful order, but by hash value. If you look at the source code:
Reference Source[
^]
and the Insert method (which Add calls) all indexing is done via a hashcode, which will not have any direct relation to the strings you are using as keys.
What I'd suggest is that you think of a different way to either store them, or access them - because even if you get code that works, you are setting yourself up for unpredictable and inconsistent results in the "real world".