I tried "-t 12 --cpu-affinity 0x555" (0b010101010101) and got a more high and stable result:
max 2300H/s, avg 2100 H/s on unlocked 2678v3
That's strange you only had 6 bits in the mask so you're running 2 threads on each CPU.
Try 0x555555. On my 6700K I get about the same with 4 of 8 threads with default mask or 0x55.