IAP AppleOriginalTransactionID is empty

We released our game with subscription added to it. For apple side verification we use apple original transaction id and we also save it for our own purpose. 33% of our users are pushing this appleOriginalTransactionID on our database as empty string. We are not able to reproduce the case locally. Game is on Unity 2021 IAP version is v4.9.4 Its happening on IOS devices only.