This issue might happen due to several reasons. The first possibility could be something with the data processing in Google Analytics. Sometimes, they encounter issues that can temporarily affect data reporting. If it’s the case, it will likely be resolved by Google in a short time frame.

Another possibility may be that the specific user IDs whose data you’re trying to fetch might not have triggered any event recently causing the dimension data of those user IDs to be “(not set)”.

Finally, it is crucial to make sure that the custom dimensions are still active and not accidentally deleted or altered. Even if you haven’t made changes, it is still worth checking out if something has accidentally been modified or removed.

Overall, it would be best to thoroughly check your Google Analytics setup, maybe even involving a professional to analyze it. If everything seems fine, you might want to reach out to Google Analytics support.