Our developers have let us know that this issue should be fixed in an upcoming update to the Clock app - so it may be worth keeping a look out for that in the near future in the Galaxy Store.
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I have a Samsung Galaxy A70 mobile phone and my device auto restarts at 3am and the alarm plays the wrong tone in the morning and not the selected sound. However, it plays the default Samsung ringtone which is very loud.
I believe this is a software error and Samsung should fix this issue as soon as soon as possible.
I have a Samsung J3 and it has recently started auto updating and then not implementing the alarm, so this issue hasn't been fixed. Nearly late for work again today, and not happy!