Sharing Externally Security Group

Hello,

We're trying to setup Sharepoint sharing with the "Allow users only in the specified security group to share externally" set to be able to share externally to anyone with the link. However it doesn't seem to be working or is being ignored. I've tried with security group and mail enabled security group and the users aren't able to share the link with anyone, the option is greyed out, the option they can share to is 'People with existing access can use the link'. However what I have found is on 'Site Permissions' on a site and then 'Change how members share' if I select Site members can share files & folders the members of that site can share to anyone however they don't have to be in the security group that's set to only be able to share. Any ideas where we're going wrong to just be able to get the security group members to be able to share to anyone?