Some standard cards like Haktos from Theros and Kaalia from m20, are registering as legacy cards in this event.
Haktos is a Human, and it’s a ‘No Humans’ event, so that’s probably the reason. Are you seeing any non-Human cards doing this?
*Edit: Kaalia Human too, of course!
Wow, good point! Thanks for that, that’s a unique rule, I totally missed it