ESP8266 + Micropython: Why do I keep getting the same value with periodic i2c reads? - accelerometer

Question

I am writing some simple code using MicroPython running on a Digistump Oak, which is basically an ESP8266 breakout board. I'm trying to understand the behavior that I see when performing periodic reads of the sensors via i2c.
The following code (which reads the value of the ACCEL_XOUT_H and ACCEL_XOUT_L registers) works just fine:
>>> from machine import Pin, I2C
>>> bus = I2C(scl=Pin(2), sda=Pin(0))
>>> while True:
... h, l = bus.readfrom_mem(0x68, 0x3b, 2)
... print (-((((h<<8)+l)^0xFFFF) + 1) if (h & (1<<7)) else (h<<8)+l)
(That print statement is just performing the conversion from two's complement.)
As expected, that prints out values from the accelerometer that change in approximately real time as I move the imu around.
But if I introduce a delay into the loop, such as...
>>> import time
>>> from machine import Pin, I2C
>>> bus = I2C(scl=Pin(2), sda=Pin(0))
>>> while True:
... h, l = bus.readfrom_mem(0x68, 0x3b, 2)
... print (-((((h<<8)+l)^0xFFFF) + 1) if (h & (1<<7)) else (h<<8)+l)
... time.sleep(1)
...I see some very strange behavior. The values returned by the i2c read operation continue to remain the same for many iterations after the imu has changed orientation. I'm at a loss as to what is going on here: the i2c read operations read from the registers on the imu, which according to the documentation are updated at the sampling rate, which in a default configuration is going to be 1Khz. I don't see anything in the code or data path that could be latching or caching these values somehow.
This is the documentation on the accelerometer registers, as found in the Register Map and Descriptions document:
These registers store the most recent accelerometer measurements.
Accelerometer measurements are written to these registers at the
Sample Rate as defined in Register 25.
The accelerometer measurement registers, along with the temperature
measurement registers, gyroscope measurement registers, and external
sensor data registers, are composed of two sets of registers: an
internal register set and a user-facing read register set. The data
within the accelerometer sensors’ internal register set is always
updated at the Sample Rate. Meanwhile, the user-facing read register
set duplicates the internal register set’s data values whenever the
serial interface is idle.
Since I'm sleeping between read calls, I'm pretty sure the i2c serial interface is idle by any definition, and I don't see anything else seems relevant to this behavior.
Do you have any suggestions as to what could be going on here?