OFI 7.1.2 Object status alert from clustered IS

I noticed the rule violation alerts for IS object status only send out when all clustered IS are down, instead of one when it detects an IS down.

For instance:

09:00 IS node #1 was taken down.
09:05 No “Problems” was picked up. “System Overview” showing green status.
09:10 IS node #2 was taken down. (The last of them)
09:12 Received 2 alert emails from rule violation.

Any advice please?