NSUserDefaults for app group suite fails to write from keyboard extension

I have an iOS app which contains a custom keyboard that can be used by any app. They share a few prefs by using an app group and a named suite of NSUserDefaults. Each target (app, keyboard) keep their own instance of the shared prefs by storing it in a property:

self.sharedPrefs = [[NSUserDefaults alloc] initWithSuiteName:kPrefsSuiteName];

And then add observers for a couple different prefs:

[self.sharedPrefs addObserver:self forKeyPath:kPrefRecents options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];

When running in the Simulator, both the app and keyboard can write to the shared prefs and both target's observers correctly respond to changes. But when running on a device (iPhone X, iPad Pro 11"), the app can write to the prefs, but the keyboard fails with:

2023-04-17 14:20:20.095600-0500 projectname[561:23591] [User Defaults] Couldn't write values for keys (
    Recents
) in CFPrefsPlistSource<0x2807dc500> (Domain: group.com.my.key, User: kCFPreferencesCurrentUser, ByHost: No, Container: (null), Contents Need Refresh: No): setting preferences outside an application's container requires user-preference-write or file-write-data sandbox access

I also noticed this when the app adds its first key value observer to the shared prefs:

2023-04-17 14:57:36.610366-0500 projectname[820:45796] [User Defaults] Couldn't read values in CFPrefsPlistSource<0x280bc1c80> (Domain: group.com.my.key, User: kCFPreferencesAnyUser, ByHost: Yes, Container: (null), Contents Need Refresh: Yes): Using kCFPreferencesAnyUser with a container is only allowed for System Containers, detaching from cfprefsd

This is supposed to work. What's wrong here?