I would also say that it is not a bug, and instead will say that you have a very balanced system for MSFS. If both CPU and GPU are maxed out, you have no bottleneck with the settings you have set in MSFS.
It is the same with RAM, I don’t understand it when people say “Program X is using 16GB of my RAM! Why so much? I only have 32GB” What did you buy it for, when it should not be used?
Yes, there is software which uses too much of a ressource for the task it is doing, but in games generally it is favourable if the game is using all available ressource to run as good as possible.