Every time I boot LinuxCNC, the Xg54 position reading increase 256

LinuxCNC is 3.4.9.
I am using the gmoccapy interface with EtherCAT bus-controlled drives equipped with absolute encoders. The values HOME_OFFSET = 0 and HOME = 0 are set in my INI file. The zero point has already been configured on the drive, yet every time I boot LinuxCNC, the Xg54 position reading increases by 256 compared to the previous session.  why?

First the Linuxcnc version you quote is invalid. What is it really?
Second, 256  is a very signiicant number for computers because an 8 bit number (1 byte) counts from 0-255 which a computer's way of counting from 1-256.
The  G54 values are stored in the var file. I could not see exactly what us the varibale number on a quick look. I would check this after you close to make sure its not being incremented on close,

My guess is that Linuxcnc will be OK but somehow outside of LinuxCNC, your encoder value is incremented by 1 X  8 bit byte.

ini配置中注释或删除这行试试POSITION_FILE = position.txt
具体作用http://linuxcnc.org/docs/stable/html/config/ini-config.html#sub:ini:sec:traj