I solved this issue. It was due to a conflict between the PICAXE documentation & the way the LCD parses data from the bus.

The PICAXE documentation said that I had to send an offset byte and then data:

hi2cout 0,("Data")

But the LCD was interpreting that leading 0 as a $00 character & printing it ...

I just swapped the 18X chip for a PICAXE 28X1, which has hardware support for I2C, instead of using the software functions. I did this to determine if the firmware routines in the 18X were generating the spurious $00 characters.

The screen's behaviour is the same with the 28X1 - the spurious $00 ...

When the screen powers on, I see a graphic of the Matrix Orbital Logo constructed out of the default custom characters.

I wrote a couple of very simple code blocks for testing and the behaviour isn't consistent. I'm beginning to think it has to do with the i2c functions in the PICAXE firmware ...

The manual I have says LCD0821, rev 2.0. I can run the LCD at 100 or 400 kHz and the strange character is still displayed. I am using 10k pull-up resistors & have checked that the system is getting 5v signals.

It seems to be that any command with the 254($FE) prefix will display the character at ...

I am using a LK202-24 (LCD0821) with a PICAXE 18X.
The PICAXE 18X has a writei2c command for sending data to the bus.

A device running at 400 kHz and 8-bit addresses is initialized with the command:

i2cslave $50, i2cfast, i2cbyte

After that, the syntax is pretty simple, writei2c requires the ...