The issue:
Screen appears about 2-3 dots dimmer on max brightness after waking from sleep for more than 30 minutes. I only noticed this a few months after upgrading to Catalina, it's been ongoing ever since.
My settings:
No auto-adjusted brightness, disabled screen temperature modifiers like NightShift/Flux, slightly higher gamma color profile for better contrast/brightness (it looks a bit faded otherwise).
My computer:
MacBook Pro 13" early 2015 mid-tier model with Catalina 10.15.4. (bought in 2017 used, no repairs or hardware issues the whole year. Great battery cycle life).
My Temporary solution:
Resetting the PRAM (cmd + opt + P + R). This fix lasts until I close my laptop and let it sleep overnight usually. Sometimes I've had success reducing the brightness before putting it to sleep. I am not completely sure if this is hardware or software related.
Edit:
There have been rare/infrequent occasions when my display would do weird things after waking from sleep. There was one incident of my display completely blanking out with a white pixelated screen for a moment (like an old static TV out of tune). A few other incidents where the display blinks black or totally freezes. It was at first a problem with video playback and hardware acceleration, but now I am beginning to believe it may be a hardware-related problem (though this Mac doesn't have a graphics card).
Thanks for your time and consideration!
Best Answer
Just figured this out for my case. I had already disabled the brightness from automatically adjusting. Every time my computer woke up from sleep however the brightness would still drop to around 50%.
Turns out I also needed uncheck the box to "Slightly dim the display while on battery power"
System Preferences > Battery > Slightly dim the display while on battery power
See the image below: