It's etherkit 1xx i suppose but with fixed PLL. : "si5351.set_freq((bfo_freq + f) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK2);"
Should be OK. I also tried Etherkit 2.0.1 with new class "si5351.set_freq_manual()" but with the same effect.
May be the cause is that si5351 is working with arduino without lvl conversion.
I dont know