Delta implements gift cards as, effectively, extremely flexible tickets (so flexible that they allow name changes etc.). Redeeming a gift card is thus effectively changing a ticket (with any residual turning into a new "ticket") and a situation where the ticket is more than the value of the gift card is just collecting fare difference (and posts as if it's a change fee, which is what triggers the credit: there isn't really a way to argue for Amex to give you the credit).

So if buying multiple tickets in one transaction, only one ticket can be partially paid for with a gift card: the others are either fully covered by the gift card (so no add collect) or the gift card isn't applied to the ticket.

This mainly comes into play if trying to book travel with kids: the Amex credit is decidedly not worth UM.

It should be noted that guest charges for the SkyClub (presumably if you had an executive membership, you'd have frequent enough other opportunities to redeem the Amex credit via the gift card route) are an explicitly allowed credit opportunity, so if traveling with someone, guesting them into the SkyClub is a good way to trigger the credit (lower effort IMO than laddering changes or getting gift cards, though it perhaps doesn't have the thrill of exploiting a loophole).