"Subscriber count dropped
We take the relationship between our content creators and their subscribers seriously and work hard to keep those numbers accurate by filtering out suspicious activity around the clock. We recently discovered a bug that prevented changes in subscriber counts from being accurately displayed publicly on creators’ channels.
The bug has now been fixed and creators’ public subscriber counts have been corrected. The bug did not affect subscriber counts reflected on YouTube Analytics, where you have had access to accurate subscriber numbers."
The above was YouTube Help from Google Support