What do you mean by this? As the setting is for external links to open in new tabs. Not every link.
Also, did you enable that setting after the others accounts were created? As it doesn’t backfill, they would still need to update their own preference, the global setting only impacts new accounts created.
I mean that if an external link is shared in the opening post in a topic or an external link is shared in a subsequent reply, the problem still applies.
I tested on new posts after I changed the setting and the problem was there (but to your point, if accounts are not updated, it would remain the same behavior). I will check for new accounts (I did a bunch of invites today I can test). I will follow up after I have a chance to check.
As a side note, if accounts are not updated with this setting, should this be considered a bug? I would have expected accounts to reflect global policy.