The problem here is identlfying exactly what the law said about your arrest record because of the PTI program. What state did it occur in, and was the charge a felony or misdemeanor? In some states, if you qualify for a program that allows you to avoid a conviction for a first offense, there will also be a provision what will allow you to seek sealing of the arrest record. If you succeed in doing that, the law may say that you may lawfully state on most employment applications (though there are often exceptions) that you were never arrested. But that may only help you with employers in that same state. In what state is the new job? And did you take whatever steps were needed to seal the arrest record? You may have a problem here if the state required you to take extra steps to seal the arrest and you failed to do that. Finally, what is the nature of your job? Some jobs the employer is allowed to know of the arrest, even with sealing of the arrest record.
You really should have nailed down exactly what the effect of the PTI program was and whether there was anything more you needed to do back when you completed the program.
Understand that your current employer is quite likely unaware that you entered and completed the PTI program. What the employer likely knows is just that (1) you said on your employment app that you were never arrested and (2) that the background check said you arrested. The employer will see that as a lie, and may fire you for that. It is not illegal to fire someone for lying on the employment application. So unless the applicable state law actually says that you can truthfully answer no to the arrest in your circumstance, you may not have a leg to stand on in keeping your job. Even then, you'd need to explain that to the employer — it won't know otherwise.