You might connected relay module to Arduino. Once power up, relay flickered in a second.

You might not like it because it gave unstable output (or unwanted output) during initial boot up.

Let check, which digital pin do you use. The problem only happen at pin 13. By default, it connected to LED at Arduino. In programming, it is known  as LED_BUILTIN.

In normal condition, while boot up Arduino, this LED will flicker in a second. There is not wrong.

This issue only occur at pin 13 but not the other digital pins.

For solution, use digital pin other than pin 13.