Can you please perform following steps and see in all three cases you are getting same values, if not then please contact us using this link https://www.mathworks.com/support/contact_us.html
Note: I have used I2C Peripheral address as 0x5c and Read Register address as 0x2a. You can use 0x55 Peripheral address and 0x3d Read Register address when you try following steps.
Please have a fresh instance of R2020b MATLAB and reboot your Raspberry Pi before proceeding ahead.
(1) Create a raspi object in MATLAB and call readRegister on i2cdev object (like how you did before).
(2) Go to putty terminal and execute "$ i2cget -y 1 0x5c 0x2a w" . Use required addresses here.
(3) Use attached myfile.c to compile and execute to see the register values, here are the steps
* In Putty terminal create a new file using "$ sudo nano myfile.c"
* This opens up a Nano editor, copy the contents of attached file and paste in the editor
* Please change the code in myfile.c to required peripheral address and register address.
* Press 'Ctrl+c' and click yes to save the file
* Now compile and build the myfile.c using "$ gcc -o myfile myfile.c"
* Run the generated binay using "$ chmod +x myfile" and "$ ./myfile"
* You should see the peripheral address, register address and data in the terminal.
MathWorks MATLAB Hardware Team