Hello Carne, Welcome to the Community. We understand the frustration of the keyboard settings not maintaining the customizations.
This article provides some troubleshooting steps that may help: https://www.samsung.com/us/support/troubleshooting/TSG01209532/
If the settings continue to change, please submit an error report through the Members app to provide the information to the developers. https://www.samsung.com/ae/support/mobile-devices/raising-an-error-report-via-samsung-members-app/