The seller is required to notify HPD of the transfer. If you don't register it, it leaves a gap in the process. It might cause problems for the seller later.
I would at least send a letter to HPD with the firearm details, seller's info, and that you have left the state. Once you are no longer in the state's jurisdiction, there's nothing they can say about the failure to register.
The letter is your choice, but personally I'd send one to tie up the loose ends for all parties out of courtesy.
I'm saying what I would do. There's nothing I can see in the law that covers this "exception." You're safe if you do nothing, I believe.
What if you move back or decide to go hunting in Hawaii? If you register the same gun as required, that gap could come back to haunt you.