There appears to be an issue with the IMU module in M5Flow. The function "Get X Acc" appears to report the orientation of the device, instead of the accelleration. Here is the python code to reproduce it:
from m5stack import *
from m5ui import *
from uiflow import *
imu0 = imu.IMU()
lcd.print((imu0.acceleration), 0, 0, 0xffffff)
lcd.print((imu0.acceleration), 0, 20, 0xffffff)
If you run the program, the displayed numbers are between -1 and 1 depending on how you turn the device.
I also tried the C/C++ example for the M5Stack located here: https://github.com/m5stack/M5Stack/blob/master/examples/Basics/IMU/IMU.ino
This example seems to run fine.