SharePoint365 folder sharing not respecting settings; cannot share externally

I am trying to share a folder (not individual files) within a SharePoint365 document library with people outside my organisation whose email addresses have been explicitly provided with access, but its not allowing it (I cannot select "Specific People" and then type in addresses outside my organisation):

Based on a lack of a link to follow, and no "External Sharing" setting is obviously visible in the Admin portal, I've followed dozens of tutorials that cover:

• Office365 Admin > Sites > Sharing Status
• Site Collections > Sharing
• SharePoint Admin > [Site] > Sharing > Policies > External Sharing

All settings are currently configured the same (correct) way - "This site can be shared with new and existing guests". 

When I try to share an individual file within SharePoint365, I see the correct options:

But folders do not appear to follow these configurations...

How do we configure SharePoint to allow folder sharing using the same rules as file sharing?