I realise I have this issue where after the phone auto restart, my setting change, etc my alarm was another sound but it changes this morning, I have a app quora which was in dark mode but it changes to white again after restart out if nowhere. I had this phone less than 1 year.
Thank you for reaching out! Have you verified all of your apps and software is up to date? There are a few things you can try to resolve the issues you are experiencing. First, verify your Time Zone and Date and Time Settings are correct. Then I recommend clearing the cache partition of the phone.
If the issue continues, place the phone in Safe Mode to see if the basic functions are still changing, such as the alarm and sounds. https://www.samsung.com/us/support/answer/ANS00062983/
Please let me know if this helps.