I’ve had this problem for a couple of months now, and Apple support insists it can have nothing to do with recent system updates. All this happened before the virus emergency, so I can’t do anything involving travel now, but I thought I’d throw this out to the group to see if anyone had a clue about what might be happening.
My iMac (Retina 5K, 27-inch, 2017) running Catalina 10.15.3 is connected via ethernet cable to a Marantz 70R11 audio-visual receiver which in turns can send the HDMI signal to my Samsung TV for screen sharing. I use this connection mostly to send audio from my computer to the Marantz, and thence to my speaker system, with the video being routed to the Samsung.
When I boot up my computer, it invariably resets the screen display to maximum brightness. I do a great deal of photo editing, and the screen must be set at the central point of the brightness range so that the onscreen image will look as it should when printed.
Every time I do anything with the Marantz like change sources, turn it on or off, or turn on or off the Samsung, the screen brightness pops back up, even if I choose “Use as separate display” on the iMac. The only way to stop this it is to unplug the ethernet cable and use WIFI to connect to the Internet. Very inconvenient and annoying. That cable connects inside my equipment rack to both a Comcast WIFI router and the Marantz receiver, but the rack is very cumbersome to deal with and I dread every time I have to pull it out and poke around among the maze of wires inside.
But clearly something is happening involving the connection to the equipment in the rack that is causing this. What I’d really like is to be able stop this misbehavior without disconnecting the wires, telling the iMac “Middle setting! Stay! I mean it!”
I’ve talked to several Mac support personnel with no luck. They’ve had me reset the PRAM and SMC several times (seems to be one step up from the old "unplug, plug back in” routine to fix a mystery malfunction), but that does nothing.
One person insisted I needed to take my iMac to the genius bar in Seattle (an expensive ferry and car ride away from where I live on Bainbridge Island). I tried to explain that the iMac works fine when it’s disconnected from the audio-visual system, so taking the iMac alone in would do no good, and not only would it be impractical to tote my other equipment to Seattle as well, I’ve been told repeatedly that Apple support will not deal with non-Apple equipment problems. He couldn’t be made to understand—kept insisting I needed to take the computer in.
It’s easy enough to slide the brightness control back to the center, but occasionally I forget and wind up wasting time editing a photo in the wrong brightness mode and have to start over—if I’m lucky enough to have kept the original. This flash-to-bright can happen a dozen times a day. I’m just about resigned to it, but would love to hear from anybody who’s ever experienced something like this and found a solution.