Promotional offer purchase fails in Sandbox with ASDServerErrorDomain 3902 after payment sheet

Hello,

I’m integrating promotional offers for auto-renewable subscriptions using StoreKit 2.

The offer is displayed correctly, the Apple purchase sheet appears, and I can start the payment flow. The sheet shows the correct discounted price and the end date of the offer. However, after confirming the purchase, an alert appears saying “Unable to Purchase - Contact the developer for more information”

When dismissing the alert, Xcode logs the following:

Purchase did not return a transaction:
Error Domain=ASDServerErrorDomain Code=3902
"No se ha podido realizar la compra"
UserInfo={
    NSLocalizedFailureReason=No se ha podido realizar la compra,
    client-environment-type=Sandbox,
    AMSServerErrorCode=3902,
    storefront-country-code=ESP
}

Test environment:

• App installed from Xcode on a real iPhone
• Logged in with a Sandbox Apple ID
• Using StoreKit 2
• Promotional offer applied using:

Product.PurchaseOption.promotionalOffer(_:compactJWS:)

On the server side, I generate the promotional offer signature exactly as described in Apple’s documentation: https://developer.apple.com/documentation/storekit/generating-a-signature-for-promotional-offers

• The signature is generated using a Subscription Key
• Signed with ECDSA + SHA256
• Uses the correct invisible separator (U+2063)
• The signature is validated locally using the derived public key and verifies correctly

The sandbox user has had previous subscriptions, which is why this promotional offer is eligible and shown.

Given that:

• The offer is displayed correctly
• The purchase sheet shows the discounted price and duration
• The signature validates locally
• The error occurs only after confirming the purchase

My question is:

Is this a known limitation or issue with promotional offers in the Sandbox environment? Should promotional offers be tested exclusively via TestFlight instead of Sandbox?

Any clarification would be greatly appreciated.

Thank you!