Check to make sure automatic brightness is disabled. Unfortunately, manufacturers keep adding it and enabling it by default, when every user I've talked to hates it. I've seen five different versions of it, so you may be suffering from more than one.
■Some monitors have it built in. It's more common on TVs, but I've seen it on a few monitors.
■Some laptops have baked in. Controlled either via a BIOS setting or with an app the manufacturer included with the laptop. e.g. Samsung called it eco mode and some driver automatically ran it every time Windows started. I never did figure out a way to disable it.
■Intel can have in their display drivers if your hardware supports it. I think they call it display power saving technology?
■Windows can have it in two different places. In the display settings, and/or in the advanced power options.
■Chrome has an auto-contrast feature which can look like auto-brightness.
Because it can appear in so many places (and sometimes in more than one), it's tough to troubleshoot. But hopefully the above list will give you some ideas of what to check.