Printers has always been a source of trouble - either it won’t print correctly or not at all. If you add a network to the mix it makes it worse.
So when my Canon MF645C started to print just some content (the pictures, none of the text on the pages) I was not surprised. I did realized that I upgraded to macOS Big Sur a few days back (jumping from 10.15 to 11.3) so this was primary suspect. A quick google (no Duck Duck really) and I found this page.
Quick fix is to download new drivers, remove existing printer, install new drivers and re-add the printer.
Problem solved easily - this time at least!