Seems to be a bug, but you can try make it work like this:
f = Quantity[55.5556, "Hz"];
Plot[Cos[2*\[Pi]*f*Quantity[t, "ms"]], {t, 0, 18}]
Plot[Cos[2*\[Pi]*f*t], {t, Quantity[0, "Seconds"], Quantity[18, "MilliSeconds"]}]
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