The hidden date tool does not have the possibility to look at just your mystery finds. When you add the cache type filter, that just affects the suggestions you are shown. It's not broken, it just doesn't have the feature that you need.
You can use Map Compare to find mystery caches hidden on a specific date but unfortunately you can't ask for "this date, any year", only one specific year at the time. That means it's a bit painful finding caches to complete this challenge.
