My experience has been that the credit is indeed reversed.
Per the AMEX T&Cs:
Statement credits may not be received or may be reversed if the booking is cancelled or modified.
Here's the link:
https://global.americanexpress.com/c...dit/delta-gold

Be aware that these bookings are usually more expensive than those not made through the Delta booking platform.
Since the credit is only issued once per calendar year, you may want to limit your use of Delta Stays, otherwise just know that you're paying a premium for the additional SkyMiles.