New Ionia Card - Zinneia, Steel Crescendo

I played vs this card, and the opponent used Blade Dance while already having this card out. Zinneia then joined the attack as she should. But then i stunned her with Malhpite's Ground Slam, and she wen't back to the bench being stunned. I did this because i knew the opponent would play more blade dance, and i wanted to stun Zinneia so she couldn't join in on the next blade dance attack. Regardless, on the next blade dance attack that turn, while being stunned, Zinneia joins the blades and attacks me and deals damage to my nexus. I thought when a unit is stunned, it cannot attack? After that the oponnent did a regular attack, and then Zinneia was not able to attack. But it still lost me the game.

Thats normal. Stun does not prevent units from attacking if the attack is triggered by a card effect. For example if you stun Irelia she can still attack by using Bladsurge to switch position with another unit. The same thing happens with Cataclysm even if your opponent unit is stunned the card will still force the attack.