When a database is in Read Only mode, there is no writing to the log and so no log backups are needed. Spotlight is firing an alarm when Read Only databases don't have a log backup run after a set threshold. How to ignore Read Only databases?
In Alarms configuration window, there is an option to ignore certain ‘Values’ where read-only databases can be added.
Enhancement request has been submitted for this feature to be included in a future release.