Looking at the end point for Account Favorites (
https://api.imgur.com/endpoints/account#account-favorites), it states that "Gallery Image OR Gallery Album, Note: vote data ('ups', 'downs', and 'score') may be null if the favorited item hasn't been submitted to gallery".
The endpoint returns all the items that have been favorited correctly, however, the items that have been submitted to the gallery are not returned with their score. The description of the endpoint would lead me to believe that it should return the scores if they have been submitted to the gallery. Some examples of items that are not having their score returned are the following ids: JOsCg, dlm9Nl5, sWZQmJk