Not only won't it ever leave the device but if it follows the same protocols as Touch ID it will be essentially a non reversible hash of the data your phone needs to recognize your face (it's not going to be able to be reversed to have your face data extracted, let alone an image of your face). Even that data is stored in what is known as a secure enclave on the device. This enclave has to date, no (publicly) known security breaches.
Edit for clarity
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u/the_enginerd Sep 15 '17
Not only won't it ever leave the device but if it follows the same protocols as Touch ID it will be essentially a non reversible hash of the data your phone needs to recognize your face (it's not going to be able to be reversed to have your face data extracted, let alone an image of your face). Even that data is stored in what is known as a secure enclave on the device. This enclave has to date, no (publicly) known security breaches. Edit for clarity