With the bundle release (.aab ) file I generated for testing, my testers are able to save games they built using their own images, but they are not able to access it, even though I can see it saved in Firebase.
When they try to access their games or any sample game saved in Firebase, the app crashes.
However, if I download the game app directly from Android Studio, the testers can both write and read the games they build.

Does it have anything to do with the way I generated the bundle release (.aab) file?

When I downloaded the game app directly from Android Studio by USB, to different devices and different tester emails, and the testers can both save and play the games they created. Everything worked correctly. I generated fresh app bundle from the Android Studio, placed it in the Play Console, then copied the link to share to testers. It still didn’t work. It still worked seamlessly when the game is downloaded directed from Android Studio.