ADB Backup of Encrypted Phone

Can anyone advise regarding ADB Backup of encrypted Moto X Style (Pure), Android 5.1.1 ?

Running "adb backup -all" connected to encrypted phone, results in a backup.ab file that is much smaller than expected, maybe a tenth of what I expect to see.

For comparison:
Kyocera Hydro Wave, Android 5.1: ~250MB to ~650MB file (backup.ab)
MXPE 2015, Android 5.1.1: ~25MB file (backup.ab)

I read somewhere that when phone is encrypted with Lollipop built-in FDE, the ADB Backup will only back up base Android system files, it does not back up apps, user data, or anything else. The reason given is that when FDE is used, the base system files remain unencrypted while everything else on internal storage is encrypted, and ADB won't backup the encrypted content. I'm not sure this is true, because the dialog that pops up on phone when ADB "adb backup -all" command is issued asks for a password to be set for the backup file - why would this be needed for a backup that contained only unencrypted system files?

One proposed solution I read is to decrypt phone to do a full ADB backup. That does not appear to be an option with MXEP, except by doing a full factory reset. Besides, even if it is possible to decrypt, do ADB backup, then re-encrypt, without a factory reset, that would add major time-consuming steps to doing an ADB backup.

Hoping to use ADB for full backup, my phone is not rooted so Titanium Backup or other root backup tools are not an option. (Don't want to root at this point, waiting on OTA update to add T-Mobile LTE Band 12 support.)

Also don't want to unencrypt at this point.

Any info would be appreciated.

TIA...