As our friend Darwin has told us in his famous theory, traits which confer advantages give that group of animals an edge. That edge is transferred to their offspring, and those without those traits do not produce enough offspring, and that new trait eventually becomes the norm for that population.
There are issues with such sensory-blocking magnetic fields, issues that nature already has a solution to. In short, creatures armed with echolocation or electroreceptors can chomp up your sneaky devils. Such solutions would make your sneaky devil invisibility neat, but not advantageous enough to confer advantage to, and therefore development of that trait may not continue.
Sharks and Rays have these organs called Ampullae of Lorenzini. They help these creatures detect electrons firing as they cause muscles to contract.
If your creature emitted magnetic/electric fields, chances are they light up like a beacon to a creature with Ampullae of Lorenzini. Not only would they light up, but these creatures can use these receptors are sensitive enough to guide the fish to its prey and chomp down on it. Turning off their eyesight-disrupting magnetic fields needs to happen or the sneaky devils do not stand a chance.
Bats use echolocation to eat up insects in the middle of the night. A similar level of sensitivity will allow hypothetical predators to find and eat that prey. As bones and organs are not really magnetic, the fields produced by the sneaky devils will not affect them. Sneaky devils will also need to blend into their surroundings to avoid such a predator.
Creatures must also weigh the possible energy expenditure of such abilities. Maintaining such a strong electric field costs a lot of energy; the sneaky devils need a high-energy source of food to afford this. Of course, they could evolve the ability to turn it on or off, but it is still a lot of energy.