Uint8Array over SPI

I don't suppose you have some kind of oscilloscope you could use to check if the data is being sent?

Nope, I'm not the hardware guy ;-)

Should be perfectly fine

I assumed so as well. I'm quite confident it is an issue with the paper itself.

You could give software SPI a go in case it makes a difference?

I did and it didn't change a thing.

I'll investigate more tonight.