Mac mini screen brightness

Maybe try turning off True Tone is available.

Or disabling Night Shift. Use Night Shift on your Mac - Apple Support

If that does not help, then you need to provide more detailed information.

What year model Mac mini and what macOS version is it running?

What external display and how is it connected to the Mac mini?

You're welcome.

So Night Shift was the problem?

If not, then you need to look for an ambient light setting in the Samsung TVs on-screen menu.

see > https://www.samsung.com/ph/support/tv-audio-video/why-does-my-samsung-tv-screen-dim-or-brighten-by-itself/#:~:text=The%20Ambient%20Light%20Detection%20function,darker%20in%20a%20dark%20room.

You will need to use the external displays on-screen menu to adjust the brightness.

Go to Apple > System Settings > Displays > Turn off the switch for 'Automatically adjust brightness"