Unable to share SPO site externally

Copper Contributor

I had a user request for a new SPO site to enable external sharing. I had previously tested this so we could be as secure as possible. 
1. Created a new security group and allowed only members of that group to share externally. 
2. Change Org-wide SPO settings to allow external sharing to new and existing users. 
3. Created a test site and successfully shared with External user with read-only permission. 

However, when another member of AD (part of group in step 1 created a new site) and tried to share externally it says external sharing not allowed. I have checked settings via PS but everything seems as expected. We are adding external users via Outlook and as we're unable to do so directly from the SPO site.

https://mydomain.sharepoint.com/sites/FirstTestSite                     ExternalUserSharingOnly
https://mydomain.sharepoint.com/sites/NormalSite                      Disabled
https://mydomain.sharepoint.com/sites/NewSite                           ExternalUserSharingOnly 

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