I have exactly the same issue as described above. I upgrade my gpu from 2070S to 4070 TI S. and since then when i do a flight in Europe my CPU temp went to 89 degrees during spikes on which is then recovers to 70 degrees ish.
i went and undervolted my 7800X3d and did some test in Cinabench. The max temp in Cinabench with the undervolt was a max of 71 degrees. But still after this test when i fly in Europe the CPU spikes to between 78 and 81 degrees (higher then the most demanding CPU cinabench stress test 
). MSFS is the new benchmark of stress test which should absolute not be true.
When i flew in the US my CPU was stable between 65 and 70 degrees but in Europe it spikes to really high temps. There is absolute something wrong with MSFS in Europe. In other games my CPU stays very cool. I cool it with a 360 mm X73 NZXT which does the job in cooling really well, but not in MSFS.