Is this a shame? 5-4-1 is an illegal formation!?

[Cat Harrison]

It is not an illegal formation if it conforms to this:

Throughout the whole match you must have:

a goalkeeper (blue)
at least three defenders (green) (DL/DC/DR)
at least one striker (red)
at least one player on both flanks.
at least four players in the opponents half (ML/MC/MR or AML/AMC/AMR or ST)
at least three players in the midfield section (AML/AMC/AMR and/or ML/MC/MR and/or DML/DMC/DMR)

One player on each flank is enough, but three players on the same flank will also lead to 20% possession.

I bolded what I'm guessing is what you ran into since you mention two formations; if you only have ML MR ST in opp's half it leads to penalty.