Note: Please note that this can happen with any work item type, anytime a Query on Formatted ID is being used.
I modified a custom list and I forgot to change the work item type from User Story to Defect. When I viewed the results, it had shown items of the work item I selected - even though none of them actually matched the actual ID of the items. Only the NUMERIC portion matched.
For example, selecting "User Story" but the query contains "DE1" which is an ID for a Defect.
Custom List will return "US1".
Release : SAAS
Looking at the WSAPI Docs, the query for formatted ID uses only the integer portion in the query.
Custom list uses the Type that is selected in order to access the correct endpoint. In this example, "User Story" was chosen as the type.
This is done for storage and performance reasons. We don't have to store the prefix for every work item. Also, a query on an integer is much more performant than on a string field (prefix + integer).