iMac 27" 2017 screen at 40% brightness

I have a 27" iMac 2017 running Catalina with a screen that occasionally does not go beyond 40% brightness. The keyboard function keys and the system preferences display slider indicate the screen is at 100%, but using either does not affect the screen brightness until I get to the 40% level, and below that it will dim further. I do not use the auto brightness adjustment option.

Sometimes resetting the SMC works, sometimes it does not. I had this problem prior to upgrading to Catalina and was hopeful that the upgrade would eliminate the problem. My hopes have been dashed.