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As far as I know, that's simply not possible. FaceID data is only ever kept on your device inside the Secure Enclave and is never backed up to the cloud, or even any local iTunes connected computer. When you reset your device to downgrade, that FaceId data will be deleted and you'll have to start again.

Again, these are the perils of using a beta.

Edit: From the Apple FaceID Security Guide

Face ID Diagnostics

Face ID data doesn’t leave your device, and is never backed up to iCloud or anywhere else.
 
Been there, done that.
If you have important messages, just back-up everything before downgrading to v11.2.6
All will be gone. You may use 3rd party app to retrieve it.
 
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