I did try HT off previously and it didn't help.
I just ran it on my 5800H CPU laptop. Same thing, kinda. On all the Intel CPUs I tried, thread 3 is 100% at times, but on the 5800H I'd eyeball and say 3+4 was hitting 100% still, but it wasn't on 3 only.
BTW I just realised I'm breaking convention if you ever used affinity in Windows. I'm counting threads as 1-CPUthreads, whereas Windows counts them as 0-(CPUthreads-1). So have an off-by-one error thrown in for free.
Edit: I'm aware the main game was somewhat single threaded, but maybe I was hoping too hard that with the graphical update they could spread the threads around a bit more.