Pdoa measure issue

hi, all

We made the AOA board by ourselves, and the measurement found that the angle value obtained when the label is rotated will be different, similar to the PDOA-Primer.pdf document, Figure 4 Path difference distortion.

As shown below, but the horizontal axis is the real pd, and the vertical axis is the 12 measured values of the label at different angles. -2 ~ 0 corresponds to -90 degrees ~ 0 degrees, and 0 ~ 2 corresponds to 0 ~ 90. Degrees, you will find that there will be a gap of 45 degrees. In this case, even if I do a nonlinear fit or look up the table, there will be an error of at least 22.5 degrees when walking randomly with the label, which will cause a very bad Positioning effect.

what reason will result in the issue ? how to slove it?