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Does your phone has a protective case, film and/or glass?
settings > about phone > diagnostics > test > test the proximity sensor
Screen protectors can often interfere with the ear proximity sensor. As a test, is it possible for you to remove the screen protector and see if the issue persists?
It's better if you narrow down the issue, remove it and test it, or you can contact your Local support to get the phone inspected and most likely that's one of the first things they'll do.
Test it in safe mode, see if the same thing is happening. It may be a third party app using the proximity sensor process.