Had the same issue today, and indeed this is not a bug. I was supposed to spy on Cassandra, I was redirected to Debbies room which was ... empty.
Looked at the code, I felt stupid. What happens is that dnir == True, in which case we cannot reach cass_flirt_debbie2. Because in fact she has a homework ongoing in amys room (staff_training_02). So indeed both quest lines cannot happen in parallel. I set dnir = True to lift this temporarly.