This is rather weird. On production, it seems that the annotation cache is always empty, so no data for any of the bug labels is generated. 

On staging, the linked bug summaries work. 

It probably comes down to a difference in the data if I had to guess. Prod uses the same requests to Monoral to get the bug queue, so I doubt it's a permission problem. 

My first guess was that maybe there's annotation data on production that breaks the Monorail query. For example, if a bug entered had a space in it, then it might query Monorail for something like: 

id:1,2,3 random text

which would return blank even if issues 1 and  2  existed since it'd search for "random text" as well. 

I tried editing all of the annotations in the Datastore manually and this problem still happens though. Will have to investigate more.