I have a playlist which used to have 1250-1300 subscribers. Today, Spotify said there were only 677 subscribers. I double-checked on a second computer, where I saw the number drop from 1249 to 677 when I logged in.
We have seen other threads relating to the subscription count not updating correctly. We are rolling out an update to the way subscription numbers are handled in the future, which should hopefully resolve any issues of incorrect subscriber counts.