Car : EA 3.9L Spac

G'day, i've been chasing this miss for a while, it was after a throttlebody clean and basically if i put a piece of paper over the tailpipe, it'll get violently sucked in when the car misses.

I've re lapped the valves, they're not leaking, i've done a leakdown test already, all electrical components are in good working order.

What on earth could cause suction in the exhaust? Any tips would be greatly appreciated, cheers.