Z-axis drift in odom VINS

[xxxy12]

I want to use VINS as odom but find it always have drift in Z-axis and only in Z-axis, which is not occur to f2m(I move my robot in 2D ground).

The result of vins like:

This cause my floor split:

Compare with f2m as follows:

Does this caused by the opticalFlow method in consecutive frame used in vins? But why the drift only appear in Z-axis? How can I solve it? Looking forward to your response.

[xxxy12]

Here is my rtabmap_vins.db