IS-IS route summarization on Cisco

I have been testing IS-IS on a lab environment. I have a bunch of level-1 prefixes in an area that I want to summarize when advertising them into other areas. the area in question has only one router exit where I do the summarization, after applying the summarization command I get the summary route entry in the routing table pointing to Null0, and the router starts advertising the summary to other L2 routers.

Now the problem is that the router is advertising the summary but still keeps including the summarized (more-specific) prefixes in its L2 LSPs.

I have tried multiple versions of IOSv and even older dynamips images, but the L2 router is always including the summarized prefixes.

Is this the way it is supposed to behave or did I miss anything ?

I used the command :

Router(config)# router isis
Router(config-router)# summary-address {prefix} {mask}