It could be defective and in need of repair but since you'll have to do a factory reset anyway before your send it in, why not try that now without restoring your backup and see if you experience the same issues.
And you don't mention the weather but sitting on the seat next to you in the sun or hot car can cause that behavior.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)
@userMp8mKu5s8q You would want to do the factory reset as suggested by @bill-e to make sure there are no software issues. However, if the phone does need repairs you can take the phone to a local uBreakiFix location for evaluation. https://www.ubreakifix.com/samsung or send a private message with the full model code and the IMEI number of the phone. http://bit.ly/2XERsky
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