ok after 2 day’s investigation, it has been discovered that:
The purchasetoken (inapp:com.mycompany.myapp:android.test.purchased) generated by inapppurchase item android.test.purchased cannot be used on server side validation as androidpublisher.googleapis.com will return error 400 for such purchasetoken.
I hope my 2 day’s work can help others with saving their time.
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