Its not that. It shows a message about 'non compatible cable'. And when I use the apple cable it quite happily charges to 100% with no quibbles. And the non apple cable is a good quality one that works fine with everything else. I suspect the ipad has been designed to be deliberately fussy.

Our household has a number of iPads and never had an issue with any non-Apple usb-c cable I’ve used to charge them with, mostly Anker branded but one or two AmazonBasics or Cable Matters brand. I’ve never seen an incompatible cable warning, my suspicion is it’s a cable that doesn’t have the right signaling to go above 5V, so it’s stuck charging at 5V and the iPad prefers to charge at 9v or 12v.

Right maybe, but why make the ipad so fussy? Why cant it chill at 5v? I am suspicious of the design decisions made here