Why is this Illegal Formation?

[RJSlow]

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.

It doesn't explain why it is now an illegal formation. And also I use long passing so i don't need a strong midfield. I have not lost using this formation this season

[mrmainstream]

I didn't lost either. That is why I don't understand why this is illegal. It is a normal 4-3-3 formation, nothing special. But they must have change the rule. I was testing this formation because my AMC got injured and after that I didn't just won all the games. As a plus I shoot many many goals as well... I was thinking to keep this formation. Now I need to change back to the boring 4-4-2 formation with 2 MC or an MC + AMC.

[t11_fan]

It doesn't explain why it is now an illegal formation. And also I use long passing so i don't need a strong midfield. I have not lost using this formation this season

I don't now why is it illegal now. You should ask support and change formation.