Thanks @MiguelR, you lead me in the right direction.
z-index was the right line of thinking, but with 3d, its translateZ() 
So I needed to add translateZ(1px); to the back.
I am not sure I needed to bother with the prefix, but I will be absolute here 
.flip-card-back {
transform: rotateY(180deg) translateZ(1px);
-webkit-transform: rotateY(180deg) translateZ(1px);
-moz-transform: rotateY(180deg) translateZ(1px);
}That took way to much of my time 
Handy reference for anybody who is interested…