That means they’ve ticked the “Hide my public profile and presence features” on their Preferences/Interface page. This will hide their usercard, and replace it with the “This user’s public profile is hidden”
You can manually replicate the topic filter for these hidden users by adding, eg.
?username_filters=jammydodger to the end of a topic address. Though this does require some user-knowledge that isn’t intuitive. (note: if triggered from within the same topic, then a page refresh is required also).
I have wondered myself whether hiding the topic filter is strictly down to the extra privacy, or whether it’s just scooped up by it being on the usercard.