Existing compression ignition engines can be modified to spark ignition configuration to increase the use of natural gas in the heavy-duty transportation sector. A better understanding of the premixed natural gas combustion inside the original diesel chamber (i.e., flat-head-and-bowl-in-piston) will help improve the conversion process and therefore accelerate the diesel engine conversion. Previous studies indicated that the burning process in such engines is a two-stage combustion with a fast burning inside the bowl and a slower burning inside the squish. This paper used experimental and numerical results to investigate the combustion process at a more advanced spark timing representative of ultra-lean medium-load operation, which placed most of the combustion inside the compression stroke. At such operating conditions, the high turbulence intensity inside the squish region accelerated the flame propagation inside the squish region to the point that the burn inside the bowl separated less from that inside the squish region. However, several individual cycles produced a double-peak energy-release with the peak locations closer to the only one heat release peak seen in the average cycle. Moreover, RANS CFD simulations indicated that the time at which the flame entered the squish region was near the peak location of the energy-release process for the conditions investigated here. As a result, the data suggests that the double-peak seen in the apparent heat release rate was the result of the cycle-by-cycle variation in the flame propagation.