Looks like there is more to SKView.showsFields. When showsFields is true, if you have an SKFieldNode.customField({}) in the scene, the custom block is called for what appears to be physics bodies to the far top right, with different positions, zero velocity, 1.0 mass, 0.0 charge.
Same, this can be easily replicated using the Game Playground, by adding an SKFieldNode.customField({}) with a block that prints out the arguments.
If SKView.showsFields is false, the block is not called, as it should be.
Feedback report has been updated but there's no update on it.