Why, in an MD-RAID1 system with a hot spare, the rebuild process does not begin automatically when a RAID leg fails or is removed.

Issue

• When a RAID leg is removed, the rebuild procedure for MD-RAID1 (software RAID) setup with a hot spare does not begin.

# mdadm -D /dev/md1 
/dev/md1:
           Version : 1.2
     Creation Time : Thu Jul 27 10:50:51 2023
        Raid Level : raid1
        Array Size : 1046528 (1022.00 MiB 1071.64 MB)
     Used Dev Size : 1046528 (1022.00 MiB 1071.64 MB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

       Update Time : Thu Jul 27 10:55:55 2023
             State : clean, degraded 
    Active Devices : 1
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 1

Consistency Policy : resync

              Name : localhost.localdomain:1  (local to host localhost.localdomain)
              UUID : 99eb22ed:b105120a:b207b85c:062fa5b4
            Events : 60

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       2       8       32        1      active sync   /dev/sdc

       4       8        0        -      spare   /dev/sda