I'm trying to lower the display brightness with xrandr but every time I do something keeps resetting it to 1 (i.e. maximum) within a few seconds. I have two displays, HDMI1 and VGA1 both running on the i915 driver. The problem happens in the same way with either display.

The command I am using is e.g.
I've tried disabling the gnome-settings-daemon Color Plugin which apparently can help with this issue (see here). I'm not sure I've actually managed to disable the plugin (I used my own method in dconf as I ran into problems with the one in the link) or how to to check but suffice to say it hasn't worked.

My distro is Debian Buster by the way.

Edit: Deleted
Edit 2: I have tried in both GNOME and KDE now and managed to check if the Color Plugin is running in both with: It is still running but only in GNOME. However I'm getting the brightness reset problem in both GNOME and KDE so I doubt the Color Plugin is the issue.

short answer:
full-featured desktop environments are likely to interfere with xrandr.
what you really want is to set brightness via the means provided by those DEs.

this a laptop?

Another few random thoughts: Is it possible there's a broken keyboard (or other input device) that is issuing a command to set the brightness somehow? Or how about some funky process like Firefox full-screen video or vlc?

Is it possible to turn on X debugging high enough to see anything useful in the logs?

Thanks for the replies, I have fixed it! The whole time it was a program called redshift (changes colour temperature at night) causing the problem. I had disabled redshift's behaviour so I assumed it was having no effect. But it was still running so apparently it was. Furthermore, I am able to set the display brightness for day and night in redshift's configuration file and have it applied automatically at start up, which is exactly what I wanted.