This paper demonstrates a new type of sub-synchronous oscillation of the grid-forming (GFM) converter, which occurs at low rather than high power levels. To reveal the intrinsic mechanism, a simplified analytical small-signal impedance model of the GFM converter is derived. It is found that the reactive power control loop (RPCL) can have a significant impact on the system stability. In particular, the voltage matrix introduced by the RPCL is the key factor causing instability in the GFM grid-connected system at low operating points. Therefore, this paper proposes a control strategy that reshapes the RPCL to counteract the negative effect of the voltage matrix by introducing a q-axis current feedforward, ensuring stable operation at any operating point. Finally, experimental results validate the correctness and effectiveness of the proposed control strategy.