I doubt there is a difference between Ubuntu and Arch here.

The array should show "active" in "mdadm --detail" output for 200ms after the
last write, and then switch to 'clean'.
So if you are writing every 100ms, it will always say "active".

NeilBrown

For some reason this array has never shown Clean status, only Active.

It isn't written to other than when NFS mounted for adding new content.

Any idea what would cause constant writing - I presume from what I see that the initial array sync completed?