The ‘back‘ face of the card has a rotateY(180deg) set on it. Both faces have backface-visibility: hidden set. Setting overflow: hidden on their parent (the card) causes the 3D transformed face (‘back‘ face) to disappear and backface-visibility: hidden to be ignored for the other.

From the spec (link):

The following CSS property values require the user agent to create a flattened representation of the descendant elements before they can be applied, and therefore override the behavior of transform-style: preserve-3d:

• overflow: any value other than visible.

• filter: any value other than none.

• clip: any value other than auto.

• clip-path: any value other than none.

• isolation: used value of isolate.

• mask-image: any value other than none.

• mask-box-source: any value other than none.

• mix-blend-mode: any value other than normal.

The computed value of transform-style is not affected.