Update: Instagram has confirmed that this is a bug, the cause of which it is actively investigating. In a statement the company told me "We're aware of an issue causing a change in account follower numbers for some people right now. Instagram users have noticed that they have fewer followers today than they did yesterday. It seems that Instagram has conducted a cull of some accounts leaving some users annoyed at their new lower follower counts. These weren't followers that added anything to the social media landscape, but they did give some people a boost in followers.

Source: Forbes February 13, 2019 12:43 UTC