When applying the age filter, this is removing people who have chosen not to publicly display their age. This did used to be if you set this to between ages, but it now does this even when you only have an upper age limit.
The filter needs to include all people even if their age isn't on public display and entering your age even if hidden, should be mandatory to allow these filters to work as other apps do. Without it doing this, you as a person filtering are missing out on that profile and as a person who has chosen not to display your age is being removed from people potentially finding you.
There's many reasons people may chose not to display their age, but in doing so, it should not penalise them by removing them from filters as this is then IMO discriminating against people who want to keep some privacy. By having the age being completed as mandatory, but an option to display or not would allow full inclusion.