IIRC, core 0 is the one that tends to service processor interrupts, which may mean that the data is unusually noisy in a way that tends to underestimate the RTT latency time.

There’s a similar weird artifact at index 64.

Core 64 is the hyper-threaded pair of core 0.

Not on that machine. That would only be true on a 128-thread system, but this one has 96 threads.