This configuration was made at the request of a distributer, and were common around 1999-2000. It sounds like the upper receiver (which is forged) either has some rough forging seams, or had some other flaws. During that period, demand was so high that everyone was making rifles with whatever they could get their hands on, and there were still one-year backlogs at many manufacturers.
If you buy it new, you can send the upper to Bushmaster for exchange, or possibly even pay the difference and get an A2 or A3 upper if you wish. Remember, though, cosmetic repairs/replacements need to be made BEFORE you fire the rifle.