You probably changed your password or your Apple ID connection on XCode has expired. That's right, this happened to me too. After doing a few account re-logins and some Xcode re-starts it started working for me again.

Not sure what your users are experiencing, but there's a bug indeed, that is causing the ActivityPickerViewController to go blank when pressing an app category drop-down, or even when scrolling up and down between the listed apps while some categories are open. While I'm curious about the standpoint of the Screen Time squad - the issue has been around for months or even for a few years already -, I'm also wondering if there's any way to catch the event and handle it in a user-friendly way.