It works (strangely) after second attempt.
I changed the region of my XZ1C as soon as I got it, never performed a downgrade, always updated via OTA and my attest key is not provisioned, so I guess it is somehow connected with customization, too.
Same thing happened to me:(. My phone's firm...
Here is version 5.9: http://www.mediafire.com/file/2cai7ejxfgnsbiz/GBWA_v5.90-2.17.296_%40atnfas_hoak.apk
I'm on the latest build with xposed and microG (nogapps). Good ROM! :good: