I am going to guess the airfield you were attacking had DFW's assigned to it?
Since they also fly in the campaign itself I wonder why you don't always get that message on any campaign mission.
Unless... static aircraft at a field are different than the ones that do the flying?
AI flights of DFW may or may not fly each time obviously, they may also not always be doing a particular mission/action.
If other craft are in squadrons in your region/date they may be used instead too sometimes.
WOFF is so incredible, and has many factors leading to millions of variations on AI flights alone. We try to get across the enormity of the campaign - I bet no other sim does this to this level.
That's why we need the logs and mission right after any error like this, please exit the manager and send the logs.
It is very hard to reproduce or even see some of them so we need those logs to fix.