Ok, I tried creating my own client ID and secret as per here: Microsoft OneDrive . Same problem, write, delete and list work but no read.
OneDrive mobile app is working fine and just signed in immediately.
Something completely unrelated to rclone though is that I tried connecting an Android file explorer app called "Solid Explorer". A working account worked just fine, but the broken account failed on the last step of the process after authenticating etc but the final "Connect" stage failed. It didn't give any further details other than "Bad Request"
So somehow it seems this one account is not accessible by third party apps.
Edit: Tried another I found on the Play Store. OneSync does seem to be able to read and write no problem.
Edit 2: This Linux Onedrive client has started downloading OK too: https://abraunegg.github.io/