Thanks for the additional details. Are you running the Fully Kiosk version specifically for Fire Tablets? It’s available in the ‘download box’ on the Fully Kiosk website:
Is there anything else running on the tablet? For example, was an installation script used? Or some sort of ‘optimization’ app? Or something else putting memory pressure on the device like TinyCam Pro?
I’m kind of shooting in the dark here as I haven’t seen this on any of my devices or seen any reports of this. It might also be worth reaching out to the Fully Kiosk developer to see if he has any suggestions.