Define "Allow only users in specific security group to share externally" for sharepoint sites only

Hello everyone,

Did someone find a solution to this problem?

I used the mail enabled security groups BUT the users in that group still can't share files externally. 

Also note that if I untick the box: "Allow only users in specific security groups to share externally", people can share. I'd like to assume that this means that the problem comes from this setting alone and not from other tenant wide configurations.