spend some time experimenting. My first fault was to print to the lcd on every loop cycle. I've read, that this can consume quite a bit of time and as @ricardocrudo wrote in the arduino example code, your code shouldn't be blocking the loop (for too long).
Right now, I have it working and my display shows the assignment label properly, as I only write to the display if something is changing. However, this doesn't really feel comforting, as LiquidCrystal_I2C is using Wire.h and it looks like Wire.h is blocking.
So, in order to avoid timing issues, we will need a non-blocking i2c display library.