When entering a new reference to an existing definition, one can type the name and a selection of definitions is offered to pick from. This list is ordered alphabetically and only the first four definitions are accessible.
Unfortunately, the list contains all definitions that contain the current search term anywhere. If the search-term is referenced in many definitions, the definition defining the search term itself may not be among the top four, so even entering the exact key does not allow creating a reference.
Simplest solution would be to improve the sorting order of the proposed defintions:
first show those definitions where the key starts with the search-term
then show those definitions where the key only contains the search-term
finally show those definitions where the search term only appears somewhere else in the definition
In addition, it may be helpful to allow scrolling through the proposed definitions rather than showing only the top four.
You're correct, the order wasn't respected. Here is the correct order, fixed in 1.7.1:
Requirement key starting with the search string,
Requirement key containing the search string,
Creation of a new requirement containing the search string,
Requirement text containing the search string,
Creation of unrelated requirements (i.e. always suggest this, at least).
Plus I've implemented a scrollbar so it's possible to see many requirements.