adb backup not working for certain app I need to get all files of a certain app (com.valvesoftware.android.steam.community), but adb backup only gives me a 1KB AES encrypted file. upon decrypting it there is nothing inside the tar archive

I tried: adb backup -f steam.ab  -noapk com.valvesoftware.android.steam.community

Upon entering my backup password (not the encryption password!) the steam.ab package is made. My problem is that there is just about 1KB of AES encrypted information in it. 

However, it seems I can back up other apps just fine.

What is the cause of this?
Developers can decide to have their apps opt-out of backups (ALLOW_BACKUP=FALSE). If you try to backup such an app via adb backup, the resulting file will contain nothing but the backup header (usually a 41 byte file). Working around that limitation is only possible on rooted devices, e.g. using the XPosed module Backup All Apps.