Cannot change RAID array check speed

Hello,
I upgraded my kernel from 6.9 to 6.17. They're both the "same" custom
config. I tried:

# echo 200000 > /sys/devices/virtual/block/md7/md/sync_speed
bash: /sys/devices/virtual/block/md7/md/sync_speed: Permission denied
# id
uid=0(root) gid=0(root) groups=0(root)

This used to work. Any ideas as to what I could have set wrong? I haven't
a clue!

Is there a security debugging tool or a log-file I could use to trace this
down?

Thanks,
David