Your system has only one graphics engine! The intergraded within the CPU (Intel HD Graphics 550)
In this case the T-CON board is dropping signal from the GPU. Which could be a couple of things!
Apple has had an issue with the newer 2016 onward Intel systems which placed the T-CON board within the main case allowing it to get cooked!
You may only need to clean the dust buildup within the fans and the heatsinks fin areas so the system is about to cool effectively. The other action is to run a good thermal monitoring app like TG Pro so you can monitor your systems temps and also allows you to set a thermal threshold to ramp up the fans (turbo).
The other direction is not run heavy apps that run your system hard.
Sadly the only way to fix this if the problem doesn’t subside is to replace the display assembly or the logic board after it has been properly tested to figure out if the signal is failing getting to the T-CON or if the T-CON its self has failed.