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The Ambitious Ring seems to be bugged.
The zombie spawns some times dies directly on spawn.
I do not have any /-1 effects in my deck, so it's not that. (And other 1 toughness zombies, like the Stitched Drake, stays alive even when the rings spawns insta-dies)
Comments
It seems like the Ambitious Ring is not the issue.
The Apron of Cecani's Legacy is the culprit.
It creates a zombie with the text "Other Zombies you control get +1/+1".
But it actually gives +1/-1. So either the text or the effect is wrong.