Check_MK 1.5.0p12 EE - Email notification Query

We have set up the email notifications on the monitoring servers. The emails are being triggered for a particular contact group based on the event which works as expected.
In the Host- and Service notifications view, I can only see that the notifications groups are being sent to all the members of that contact group but I see that the notifications are split which I do not expect. So, in a team of 10, 8 people are on 1 email distribution list and the rest 2 are on a separate list. I would like all of them in a single list. Can you advise how to fix this?

I would like tu suggest to send to a special mailgroup address instead of User in a contact group.
In the rule you need to set the “The following users” to your mailgroup.

Regards, Christian

Thanks for your response. Unfortunately, that is not an option for me.
I did the migration from 1.2.6p15 to 1.5p12 and everything works except this splitting of mails into two distribution list.

It was working as expected in 1.2.6p15 but I am not finding any option to configure this behavior to stop the split.

Is this somehIng to do with synchronous or asynchronous setting?