Time Machine says volume is encrypted when it isn't

I just got a new MacBook Pro and used Migration Assistant to migrate from a 2015 MBAir (both running latest Mojave 10.14.1). I turned off Time Machine backups and logged out of all stuff (iTunes, iCloud etc) before migrating. It went very smoothly. Once I turned on TM backups again on the MBPro, TM asked and I clicked on the option to inherit the same backup. As the backup started it warned me that I was backing up an encrypted volume to an unencrypted backup (see image below). In a rush to a meeting, I clicked “Close”
and packed up the laptop. Once I stopped to think about it, I realized that my MBPro volume should not be encrypted. So I checked in Preferences and, indeed, FileVault is not turned on for the volume in question (or any volume attached to the MBPro). Google-ing this issue turns up queries about this same message and no answers. Today TM went to backup to a different volume and the message popped up again. I clicked “Settings” this time, it opened Preferences. I saw all was normal and closed Preferences and it happily continued to back up. I rely solely on multiple TM targets for my backup strategy so I’m wondering if TM really thinks an unencrypted volume is encrypted. Hopefully this is a minor bug just related to the message and not something deeper in TM or Mojave. Searching talk.MPU also finds no hits but I was wondering if anyone else had this message and figured it out?

One possibility: my understanding is that at a hardware level, the drives on T2 equipped Macs are always encrypted (this is a feature of using the T2 chip as a disk controller). All turning on File Vault does is change the password for this encryption from something the OS knows to something only you know. Perhaps Time Machine doesn’t realize this and is seeing the hardware level encryption and thinks that you have FileVault turned on?

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Aha. Thanks, that solved it. I originally googled using the message text which usually works but google-ing “T2 encryption” provided an Apple Support hit for a page that explains the T2’s role in doing hardware-level encryption of the data on the built-in SSD ( https://support.apple.com/en-ca/HT208344 ). Further down on that page, a link about TimeMachine leads to another page with a recommendation to encrypt backups. No mention of the warning message but that must be what the message refers to. My TM targets are the internal drive on an Airport Extreme and an external USB 3.0 drive connected to the same AE. I’m not sure what, if any, performance hit there will be if I encrypt the backups so I will probably leave it as is for now until I can connect a spare drive and performance test a full, encrypted backup vs a full, unencrypted backup.