So this points to it potentially being an issue with your iOS device/device setup rather than Brave specifically. If you are seeing this white flash when Brave is closed and you try opening other apps, the issue is likely to do with the system itself.
Can you confirm that, with the Brave browser app fully closed on your device, if you try to open other apps installed on your device you still see this white flash?
Additionally, it looks like this particular issue has indeed been reported before but also should have received a fix – looks like it was related to the dark theme being used:
Can you please tell me what version of the iOS Brave app you’re using at this time?