Automatic re-train is a "feature" you cannot turn off, cannot turn on, have no visual feedback that it occurs, and ultimately weakens the security of your device.
If this is what is causing Face Unlock to fail then that's still just as much Apple's responsibility than any other methodology. You also have no way to knowing how much re-train has occurred since Apple doesn't provide that information.
https://support.apple.com/en-us/HT208108
To disable Face ID, open Settings > Face ID & Passcode, and tap Reset Face ID. Doing so will delete Face ID data, including mathematical representations of your face, from your device. If you choose to erase or reset your device using Find My iPhone or erasing all content and settings, all Face ID data will be deleted.
It doesn't bother me that she can unlock my phone, but I bet the facial model my phone has for "my" features is a weird hybrid.
It is pretty common for spouses to have a finger added on their spouse’s phone for unlocking it with Touch ID, e.g., when you’re driving and get a message, you can give it to your spouse who can reply.
Many parents also have a finger added on their young children’s phones.
None if these scenarios are really possible with Face ID, because you’ll end up training Face ID with two faces. And while phones are usually used by only a single person, it is not an uncommon scenario to have a secondary user.
Anyway, if you want to have a shared phone like this, get one that has Touch ID. Apple offers both.
Or trust that Apple does this intelligently and doesn't train the facial recognition in this case when the face is too far out of bounds. As other commenters mentioned, I'm pretty sure that's how it works. It's not training based on your wife's face unless she looks a lot like you. And in that case, you want her to be able to unlock your phone, so isn't it a good thing?
And if you are one of those couples that look alike enough to be able to train the phone, why do you care? You already gave your spouse full access via your passcode.
She can change the music on my phone while driving, look up stuff for me to enter into maps, and do everything else with it.
Not arguing this is ideal, but it works and avoids retrains, although Apple have publicly stated retrains only occur if they believe the face matches above a base threshold, which makes it much less likely a female face will retrain on a male Face ID user’s phone, and vice versa. It doesn’t retrain on every passcode entry.
https://images.apple.com/business/docs/FaceID_Security_Guide...
If you give your passcode to someone with extremely similar facial features, you should expect that they'll be able to unlock your phone.
This would certainly make some edge cases (for example if you didn't use your phone for a while => changed facial features) less "magic" because you would always have to enter the passcode - but a simple "retrain" should then solve the problem... am I correct?