thanks for confirming this as well, so adding a found version. Next step would be to check the latest 5.10.y upstream version. If it's an issue there, next to report it upstream.
Just to understand that correctly - shall I wait for the next update for the kernel in buster-backports and try that one? I tried 5.10.40-1~bpo10+1 amd64 which is currently the latest one in buster-backports. spi