OnOrderConfirmed not triggering

Description
While migrating a game from Unity IAP v4 to v5, OnOrderConfirmed never fires. Because the callback never triggers, the consumable is never consumed server-side.

Initial setup

• Unity Editor version: 6.3
• Unity IAP package version: 5.4.0
• Store: Google Play
• Test project: A new, minimal scene created specifically to isolate this issue, containing only:
  • A Codeless IAP Button (initiates the purchase)
  • An IAP Manager component (handles the purchase callbacks)
• Codeless IAP Button configuration: "Automatic Confirm Transaction" enabled, so OnOrderConfirmed is expected to fire automatically after purchase.
• Automatic product initialization enabled for the test.
• Callbacks wired up: OnOrderPending, OnOrderConfirmed, OnFailedOrder (each simply logs to the console).
• All products in the game are consumables, and all product IDs match the corresponding Google Play product configuration.
• The same products work correctly in the current production build, which is still on IAP v4.
• Also reproduces on Coded IAP.

Reproduction steps

• Create/open a project on Unity 6.3 with Unity IAP 5.4.0 installed.
• Set up a scene with a Codeless IAP Button (Automatic Confirm Transaction checked) tied to a consumable product, plus an IAP Manager with OnOrderPending, OnOrderConfirmed, and OnFailedOrder wired to logging methods.
• Build the game for Android and install/run it on a device (not tested in-Editor).
• Tap the purchase button to open the purchase flow.
• Tap "1-tap-buy" to complete the purchase through Google Play.
• Watch device logcat output for the confirmation log.
• (Also reproduces on Coded IAP)

Expected behaviour
After a successful purchase, OnOrderConfirmed should fire automatically (since Automatic Confirm Transaction is enabled), logging "Order Confirmed." This confirmation should acknowledge/consume the order so the consumable product can be purchased again immediately afterward.

Actual behavior

• OnOrderConfirmed never fires in the build, even though the callback is correctly wired and Automatic Confirm Transaction is enabled.
• Because the order is never confirmed, the consumable is never consumed.
• Google Play's Order Management console does show the order as recorded and processed on Google's side.
• Since the app-side acknowledgment never happens, the player cannot repurchase the consumable — the store returns "You already own this item," which triggers OnPurchaseFailed with a DuplicateProduct error.
• Roughly 5–10 minutes later, Google Play automatically refunds the unacknowledged order.
• After the refund, the player can start a new purchase, but OnOrderConfirmed still fails to fire, so the same cycle repeats — the purchase flow is effectively non-functional.
• The issue reproduces identically with both Codeless IAP and Coded IAP (manual init + scripted callbacks), indicating it isn't tied to one integration method.

Logs and Links
Forum thread: https://discussions.unity.com/t/onorderconfimed-not-triggering-iap-v5-4-0/1732315