In FDTD methods there are no infinite boundary conditions, at least not yet in OptiFDTD. However, it is possible to perform a far field expansion.
A couple things may be happening and I say either the field is changing because you are still in the near field regime, so the waves are interfering with each other or maybe there is a problem with the boundaries. In your case I’m guessing you are using a combination of periodic and absorbing boundary conditions.
Could you attach your FDTD project file so I can take a look?
Regards