Wrong dose deposited in DICOM because of direction?

I have built a symmetric patient structure and located it correctly. When the direction of primary beam is positive, I can get a proper dose map, however, something will be wrong in negative condition. When the direction is negative, first one or two layer will have much more energy ddeposited even its material is air. At the same time, interface area will lose energy and nearly 0 Gy in them. In a word, the same setting cause different results in opposite direction. It really confused me for a long time.

If you know something about it or have any suggestion, I would be grateful.

Thank you!