Not a bug, the data is nothing new. It just was stored somewhere else previously.
If you suddenly have a issue with tracking data then you have other issues in your hosting.
Before the last update the data was stored in a separate database table.
No it’s stored via the transient API – Same amount of data, same amount of read/writes, same size data – Just managed by WordPress instead of AdRotate (Pro).
That said – the next version will include some cleanup functions for expired transients, apparently WordPress has no cleanup features for that.