Monitor turns off for two seconds after login

I have a mac mini 2018 model that runs mojave with an external monitor connected via hdmi output of mac mini. After login with my credentials and when the progress bar is loading my monitor turns off for about 2 seconds and then it turns on again.

Do you have any ideas about what could be happened?

This is standard behaviour with the built-in HDMI port of the Mini 2018 - the governing circuitry has a 'security interlock' controlled by the T2 chip, and it gets toggled when the machine moves from the PREBOOT OS (which is where your main login window actually lives) over to the standard install.