Can't find the third party secure keyboard element in Android

I am using Appium v1.6.5 to do Automation. There is a third party payment page, that will pop up a secure keyboard. Automation needs to tap the number board to enter some number. But when the keyboard popped up. I can’t find the keyboard elements neither in Inspector nor in code (driver.getPageSrouce). But in the same page, appium can find all the keyboard element in iOS simulator. So now I can finish automation in iOS but not in Android.

Here is my environment and screenshots:
Appium v1.6.5
iOS 10.3
Android 6.0.0