Comment 6 for bug 1717176

That would likely be the first md_check_array_state call.
 sync_action = md_sysfs_attr(md_devname, 'sync_action')

It checks if the MD is idle or if it has to wait for a sync to complete (or to stop)

Now this is an upgraded maas, maybe the old DB for the node's disks create some odd config.
So release + re-commisioning (and let it forget all old config)