According to reports, it appears that a bug in iOS 14 is causing these default apps to reset whenever your device is restarted. This means that if you’ve chosen a different browser or email client to be your default app, once you reset/restart your device, those defaults are reset and you’ll be back to using Safari and Mail as the default browser and email app respectively.
It is unclear what is causing the issue, but hopefully this is something that Apple is working to fix. The company recently released the iOS 14.2 beta to developers, so hopefully the fix is part of the update. In the meantime, there is no workaround to the problem. The only way is to avoid resetting your device, or get used to having to manually change your default apps every time your device restarts.
It seems rather annoying and some are wondering if the surprise announcement to release iOS 14 could be the culprit, where some developers are complaining that Apple did not give them enough time to get their apps ready for the update.