Hi there, thanks for your feedback.
I managed to fix the issue by removing the following line from the manifest file,
<uses-sdk android:minSdkVersion="9" android:targetSdkVersion="28" />
I am not really sure how does this fix this issue but I'm now finally able to handle the back button on the Mi A2 lite and Samsung Galaxy S9. I got very suspicious after I wasn't able to handle the back button even by removing all my game code and just keeping one class so I knew the handlers weren't the problem. After that I removed all ANE extensions but still no luck. This is why I got suspicious towards the manifest file and I started editing/removing different entries. Turned out the element <uses-sdk> was causing this issue, I'm not sure if just changing its attributes minSDKVersion or targetSdkVersion values might work as well instead of just removing it. What do you think?