At the moment there is an issue on these high core-count systems. We hope to fix this in an update soon. However, you can suggest Terragen to use more threads than it detects automatically, and the OS will distribute those threads without Terragen needing to be aware of the hardware. One way to do this is to enable an override on the Startup panel in Preferences (set the desired number of threads, although it calls them cores). Thats a per-machine (per user) setting. If rendering on a network where you want to control the number of threads for each computer there's a command line option, e.g. "-threads 88". As a last resort if you need to force the number of threads on many computers but can't change the command line, it can be set in the render node on the Advanced tab. There you can set both min and max to the number you want. This is stored in the project (scene file).
Bear in mind that TG might not be as efficient at 88 threads. In some cases it might be slower than at 44. This can depend on the contents of the render.
Matt