The main effect causing degradation of the overlap between LO and reflected beam form FC when it is ON and OFF resonance is the relative phase change between TEM 00 (resonating the cavity) and HOM (not resoanting). Raffaele did a quick calculation to show this and the numbers he found are in rough agreement with those we observed. I attached a pdf with a similar calulation and a plot that shows the overlap degradation as a function of the not coupled power into FC.