We've seen a change of feed hits by Apple since the last Apple Podcasts updates, leading to a decrease of subscribers count on this client (you can see it as well in your feed dashboard by clicking on the Podcasts clients menu item).
It appears that since the last update of Apple Podcasts the client app (which fetches the RSS feed) doesn't refresh as often as before, therefore you can see an important drop. Currently we're showing you a 7-day sum of unique hits but maybe it's not the most relevant figure anymore.
What we will do is spread the 7-day unique subscribers count to 30 days in order to have a better overview of infrequent clients. We could also look at integrating unique downloading IP addresses as subscribers, but that would work only if you have download tracking enabled on FeedPress.