Bug or feature is debatable
Basically it used to be far worse in previous patches, there could be no reaction to shooting down 1 of a formation at all.
This behaviour usually occurs when the formation concerned decides to return to base, it's 'awareness' seems to go to zero and they fly on their way as if on their way to an airshow.
This kind of behaviour was historically possible- eg 501 Sqn had 4 ac shot down in a couple of minutes by 1 member of JG26, but the 'RTB dummies' are a bit too dumb for my liking.
Other offline campaigns
http://airwarfare.com/sow/index.php/...line-campaigns
or Desastersoft for paid addon campaigns
http://www.desastersoft.com/en/news.html
Official development may have ceased, that doesn't mean 'community based' development has