Hi Ben,
Is there any special processing being performed during the screen transition?
A white flash during screen transition is frequently a problem caused by heavy use of images, background images and gradient backgrounds which can be computationally intensive to process and can cause such issues on some devices. To isolate the cause try systematically simplifying the app by reducing use of graphics to solid colors. And if you have very long scrolled “next” screen you may try shortening it and see if it improves matters. Make a back up of all your design files before you start tweaking.
1) What device(s) and OS version are you testing your app on?
2) What type of screen transition is used for the screen(s) with the flash?
3) Can you post a screenshot of the current and next screens where the transition flash occurs?