SerOut txout, n2400, [ $aa,Manch ] is only sending $aa followed by the low byte of Manch.

SerOut txout, n2400, [ $aa,Manch.LowByte,Manch.HighByte] will send both bytes of Manch.

Modify the receive routine also. SerIn rxin, n2400, [$aa], Manch.LowByte,Manch.HighByte.