2 parallel planes, the front one is rotated, when view is rotated, portions of the plane on the back is visible through the plane on the front Any idea why the purple plane is showing through? These planes are parallel. Using Eevee. If I create this topology fresh I don't see the issue. But in this particular blender file, I somehow arrived at this problem and I don't remember what all I was trying. Which setting may be causing it?

good file
bad file
This is called z-fighting, and it's caused by blender not being able to properly decide which one to draw in front of the other.  You need some difference in the z coordinate, and if it's not large enough this can still happen at great distances due to floating point precision funkiness.