If you have Face ID enabled on your iPhone, setting a passcode only for the Hidden album may not be feasible. This is because Face ID is often used system-wide for authentication, including access to the Photos app's Hidden album. As a result, the system prioritizes Face ID over a manual passcode for convenience.
To work around this, you can try disabling Face ID entirely on your device. This may force the system to fall back to using the passcode for authentication, including when accessing the Hidden album. Keep in mind, though, that this will affect Face ID across the whole device—so you'll also need to use your passcode for unlocking your phone, authenticating apps, and making purchases.
If you're looking for more granular control—like Face ID for unlocking the phone but a passcode specifically for Hidden photos—unfortunately, iOS doesn't currently support that level of customization.
Suggestions and Feedback for improvements --> Apple Inc says; "We read all feedback carefully, but we are unable to respond to each submission individually."
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