iMac 5K Retina brightness keeps resetting (Automatically adjust brightness is off)

Apparently there was a thread for this that started back in 2011 for a MacBook and a few respondents reported issues with older iMacs also. That thread ceased in 2013 with no resolution.

The problem is that my work computer keeps readjusting its brightness to what appears to be the default setting. For me, this is particularly annoying as have an external display attached to this iMac, a Dell P2715Q, and I calibrate the displays so that everything on screen has consistent color, brightness, etc., across displays. Note I use a DataColor Spyder5 colorimeter to calibrate my displays in place of the ColorSync utility in the Display settings. Within a few days of calibration, the iMac’s display is noticeably brighter than the Dell display.

While my calibration software wants top set the brightness level to 200 cd/m² based on ambient room conditions in my office, but the iMac keeps going back to a brightness setting that is in excess of 400 cd/m². Interestingly, I have the same iMac at home—late 2014 iMac Retina 5K—, which is also calibrated, and it does not appear to have this problem either running from its internal fusion drive or the clone of my work computer on an external SSD. As I do not have an external display at home, there is no basis for comparison, but when I check the brightness, it is not at about 80% max as it is when it resets on my work iMac.

Based on the older thread this seems to be an issue with random Macs. Obviously, needing to recalibrate every few days instead of once a month is quite a hinderance. The entire reason for my purchase of the Spyder5 was to cease having the appearance of windows and documents dramatically shift between displays. Any ideas?