I have had this happen intermittently since I got my MacBook Pro 16 inch 2 weeks ago. I have it hooked up to the LG Ultrafine 4k (original 20 inch model) and a Thunderbolt 2 Lacie 2big. I have talked with Apple Support for this issue on two occasions, and they just did the standard PRAM and SMC steps. After doing these steps, it seems to be ok for a few days then it comes back. I cannot replicate or find any pattern as to when it happens (though it seemed to be more at night). Apple Support said next step would be to re-install the OS, which I do not want to do right now. It's an annoying problem, and I fix by going to settings and selecting turning automatic brightness off then on. I hope they find a resolution that doesn't involve wiping the machine and starting over again. Also, Apple Support said they have heard about it before...so fortunately we are not alone.