In this article, a small-signal model of grid-forming (GFM) converters that takes into account the presence of ac shunt capacitors in the power grid is presented. It is revealed that the inclusion of shunt ac capacitors in GFM converters leads to the emergence of two new resonant peaks in the loop gain of the active power control (APC) loop, in addition to the fundamental-frequency resonant peak that was previously identified in literature. Further analysis based on the equivalent APC considering P/Q coupling has confirmed the same destabilization effect of ac shunt capacitors by introducing two extra resonant peaks. Based on the insight, it is suggested that the active damping control needs to be adapted to effectively dampen all three resonant peaks to ensure the stable operation of GFM converters. Finally, simulations and real-time simulations are carried out to corroborate the theoretical findings.