Point 1: NPT will not give you exactly 300 K and 0 atm at any particular step. NPT will give you the correct averages when the system is equilibrated. Small systems have big fluctuations in pressure so it is even more likely that what you will see is –800 atm followed by 800 atm for an average of 0.
Point 2: Your system may not have been equilibrated yet. You set a relaxation time of 1 and ran 10. Maybe that’s good enough and maybe not. Have you plotted temperature and pressure as a function of time and checked for drift and periodicity? The parameters you are currently using may or may not actually be appropriate for your system. The only way to check is through trial and error and plotting the results.