Conformal ECO use cell out of spare(NO_MAP)

When I use conformal ECO and read the log file, I found that eco logic uses out of spare and NO_MAP cell.

Like below report:

cs_sum_dis_reg        NO_MAP                                                      OUT_OF_SPARE   DFFSX1
lowpwr_en_final_reg   NO_MAP                                                      OUT_OF_SPARE   DFFRHQX1

Actually our sprecell have FFs cell like below:

   DFFNSRX1 DTCELL_spare_dffax2  (.Q(n16),.QN(), .D(logic_0_net), .CKN(logic_0_net), .SN(logic_0_net), .RN(logic_1_net));

Since our sparecell FFs are clock negative edge trigger(CKN) and the metal logic in rtl are always positive edge trigger. So when we do manual ECO, generally we will invert the clock and send to sparecell FFs CKN port.

I hesitate if the conformal ECO can't match the positive/negative edge FFs in ECO.  So the tool choose the out of spare cell to realize it.

Pls help on this question.