Many iPhone X users have found themselves unable to use Apple's Face ID to approve family purchases, resulting in them having to enter their passwords manually whenever another user on their account tries to buy something.
Per the Verge, the lack of Face ID verification affects all transactions on iOS' Ask to Buy, the Apple system which allows parents to monitor what their children are buying online. Users aren't having to enter the passwords just once per session, but each and every time someone charges the linked account.
According to Ars Technica, users seem particularly frustrated that Ask to Buy used to support Touch ID, which has been removed in the iPhone X -- and it's particularly odd that Face ID suddenly fails to work for Ask to Buy, given that Apple introduced the new feature so that it automatically replaced Touch ID on many third-party apps. Threads on the official Apple forums show some users are getting slightly frustrated having to re-enter their passwords several times in a row.
One possible explanation for the issue is that Face ID can be fooled by close family members like children and siblings, though the odds of that are probably overstated -- and that would require having access to the phone anyhow.
As the Verge noted, the Face ID feature simply goes unmentioned on Apple's web page for the feature, so it's also possible that the company simply hasn't gotten around to enabling it yet. This is more of a minor annoyance than a huge problem, but Apple's had some practice with lots of minor annoyances adding up to huge problems with iOS this year.