Sometimes system bugs ruin the best features. For example, in iOS 14, Apple finally allowed changing the default apps, but a bug with the feature causes problems.
Users found that after restarting an iPhone or iPad running iOS 14, the system automatically changes the default apps back to the standard ones. If you installed the default Google Chrome browser, then after restarting it will be replaced by Safari again. The same thing happens with third-party mail clients.
Also read: How to Change the Default Browser on iPhone and iPad
This means that you will need to manually change the default apps after each device reboot.
This is probably a system bug that Apple will fix sooner or later. It is not yet known when the first update for iOS 14 will be released. If this problem attracts attention, then we can expect version of iOS 14.0.1 next week.
Fortunately, few of us reboot our smartphones often.