Thanks! That really does seem to be the issue and I wouldn't have known about this, had I not asked. I will try this out and will update the blog post in ~8 hours time.
Given your CPU graphs, and the large number of cores, I expect musl's allocator simply has very poor behaviour with respect to multithreading (e.g. limited or no threadlocal arenas, size-classing, etc…) leading to a lot of crosstalk, extreme contention on allocations, etc...