Unless I'm losing my marbles, I distinctly recall having spent much time setting a more complex off plan than simply having every personnel and play style (OR/IR/SP/MP/LP) an equal weighting for each down distance etc!? Is there some way that I might have reset this?!

If you select too many plays on offense or defense, or scout too many/too few plays on offense or defense it will re-set your gameplan.

Oh right maybe I selected too many on off /def game plan.
I notice it on scouting as the screen jolts down to accommodate the warning message
Cheers :)