If a login to MaxScale fails and user account fetching is not limited by "users_refresh_time", MaxScale fetches user accounts and grants from the server. MaxScale then checks if the new data differs from existing data and updates its cache if yes (updating can be a somewhat heavy operation so best done only when needed).
The comparison operation does not correctly detect changes in the "mysql.db"-array and thinks that the new data is identical to the old even when not. Thus, the data is not updated. Changes in other grant types (e.g. table-level or a new user) are detected properly and will cause the entire grant database to update. Will be fixed in 2.5.20 & 6.4.1.