I am completely guessing here but could it be due to the fact...

I am completely guessing here but could it be due to the fact that an ipad/iphone etc doesnt have "proper" GPS - its actually assisted GPS which may need your phone's electronic compass to function correctly. This, of course will be impacted by a magnet.

A proper GPS unit like you would find on a boat/plane is not impacted by magnets as far as I am aware. After all they are usually mounted next to a traditional compass which wouldnt make much sense.

To reiterate before someone bites my head off - I am completely guessing here...