String entities are not good candidates to use in a composite entity.
The basic premise with a string entity is to extract anything and everything, with the main challenge being knowing where to start and stop. Within the current composite entity processing that means that it considers the entire utterance to be a valid value.
Now there is an argument to make that a string entity in a composite should hoover up what is left over, but that is not currently what happens.
The composite entity is more geared towards “structured” data, so instead you will need to use a more closed entity type like a List of Items or a Custom entity with a concept.