I first saw this on https://community.wanikani.com this morning, and was able to reproduce here on https://meta.discourse.org just now.
On latest iOS in Safari I’d open the menu for changing the tracking status of a topic through the timeline view (this bug does not happen when using the tracking button from the bottom of the topic). Once I select Watching/Tracking/etc., the page locks up and I can’t close the menu or navigate away from the topic. I cannot even refresh the page. The only way to continue using the instance of Discourse is to close the tab and open a new one.