Why is this Illegal Formation?

[t11_fan]

Copying my previous post from another similar thread.
Basic of possession in football is (central) mildfield. You have only one player in the middle so it's obvious that he can't keep ball without help. Switch your one of your ST with mc or maybe amc/dmc.
You haven't done anything wrong, they changed calculations.
I can't remember last time I used formation with 3ST. I usually play with 5 or 6 mildfielders, sometimes with 4.