Bug in tactic #20794

Regarding lichess.org/training/20794

The winning sequence is 1. Qh5+ Ng6 2. Qxg6+ Ke7 3. Nd5+ Ke6 4. Ng5+#
Instead of playing Ng5+# I played a mate-in-1 Nxc7++ and it was marked incorrect, rather than "keep going". Seems inconsistent to penalize a mate that is only one move slower than optimal.

4.Nxc7+?! Ke7
It's not mate.

Yes, it is mate after Qg7+ Rf7 Qxf7+#
(although you are right, I mistyped and meant to write mate in 2 rather than mate in 1)