Unable to Share with External Users

I did a bit of testing on this out of curiosity, and found (at least from the sites I tested in my tenant, not necessarily a thorough examination) that external sharing works via the UI settings on sites I created prior to the time when a new site created an O365 group. The sites that are "Public Groups" all seem to not be allowing external sharing (though I have it enabled at all the levels, as you did), unless I use the PowerShell cmdlet. 

Since those are the same sites that don't show up in the list of Site Collections in the SharePoint Admin center, I hope when we get the new Admin Center (that shows the "groupified" sites as well), that maybe the external sharing settings will propagate correctly....