I tried to reproduce the issue but I was unable. I used the below methods to check if the application had been whitelisted, and it returned true.
PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
boolean status = pm.isIgnoringBatteryOptimizations(packageName);
Log.d("Optimization Status : "+status );
Is there anything indicating that the application is in Doze Mode other than that it does not respond to FCM? Could you please send us a dumpState after reproducing the issue?
Also, if you look at the android documentation on Doze Mode here, it states that FCM messages are optimized to work with Doze and App Standby modes.