HI @Nikos1987 Firstly can you check if you have Auto Power off selected, TV will turn off in this Mode if no user activity is selected for a period. The other Eco modes have energy benefits but on the other hand does effect the brightness. Ensure you have the latest firmware update on Samsung support 2303.1 https://www.samsung.com/uk/support/model/QE55Q700TATXZG/
However if this is not the reason try a soft Reset, press and hold the power button for around 5 seconds until the Samsung logo appears.
If still results in a problem I would advise disconnecting external devices ,ensure afterwards source is set to TV. Now unplug the Tv at the mains for 5 minutes , connect your Devices again and test to see if the issue is solved.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB One Ui 4.0,Android 12 Official Version.