Following this post and this and after thorough investigation I can declare the cause of the problem of reduced monitor brightness is that after some time of inactivity, system prepares to bring up screen lock. Just before that it reduces monitor brightness to 70% of its current value. Then when it is supposed to go back to the earlier value, it won’t. Either the DDC command is not emitted or is ignored by monitor. I noticed DDC commands don’t work that smooth when I played with ddcutil.