Hi there. The Illusion on my Delta Liepard is bugged. It works properly at first glance and it shows up as the last Pokémon in the party, but its name is always Thundurus. It somehow saves the name of the first one it mimics (in my case, the first time Illusion activated, it mimiced Thundurus). So, if I have Stunfisk as the last Poké in party, it shows up as Stunfisk, but its name is Thundurus. Also, when it uses a move it always says "Liepard used x" even if Illusion is still up.
Nothing big, but thought I'd report this. Don't know if this makes the enemy Pokés "see through" the Illusion, I'd think it doesn't.