I remember investigating and finding all the problematic IPs had requests evenly spread. Dozens of IPs from China, Singapore, and various South American countries. All of them had approx 80k requests in the period I was looking at, as if they send a few requests with one, then the next, then the next, rotating through then back to the start. Avoids rate limits I guess.
I am considering adding Iocaine as a first line of defence before Anubis, and seeing if that helps