Thank you for the input. The shop 1 mile from my house has springs in stock for $3, so I can definitely purchase a proper spring before firing with the cut spring.
Being that the 450 upper cycles on the BRO lower, this is why my attention is drawn to the buffer and spring. It also cycles on the M&P15 lower.
With all buffers being identical weight in the 3 lowers, the spring has my attention as the likely culprit, but I am far from an expert. I did notice the spring was 1/2" longer and stiffer than the two lowers that allow the ammunition to cycle, so I cut it, as I had read that this was an acceptable practice.
Since I will be visiting the parts store anyway, what length spring and weight buffer do you recommend for the 450? My plan is to have this lower specifically setup for the 450 and not interchange it.