Nuendo and Cubase started to disable true multicore rendering. Especially when bouncing offline.
The only way to get all your cores active is to use AsioGuard which buffers everything all the time ahead using all available cores. If you disable AsioGuard, there is basically broken Multithreading with no more than 8 core usage at best.
Exporting a project offline might deactivate Asioguard by design.
The reasoning for disabling multithreading was something about stability issues with super high core count machines that started to show up with the introduction of the first Threadripper and such.
After a few temporary registry fixes that worked for me and many others, steinberg went ahead and disabled half the cores completely. DAWBench has also removed steinberg software because of that.
My thechnical reasoning may not be 100% right, as i am just paraphrasing what i remember from some DAWBench blog that i found while figuring out why my TR CPU was not fully utilized. The temporary Regfix a few years ago worked great for me but was removed as the engine was changed with Cubase 10.5 or 10 at least.
Here is hoping they’ll fix it in the future “again”.