How do I make a spare device active in a degraded mdadm RAID5

A bit of history to start with. I had a 4 disk RAID5 and one disk failed. I removed it from the array and had it in a degraded state for a while:

mdadm --manage /dev/md127 --fail /dev/sde1 --remove /dev/sde1

My data requirement suddenly dropped so I decided to permanently reduce the array to 3 disks. I shrank the file system to much less than the new array size then:

mdadm --grow /dev/md127 --array-size 35156183040 # reduces array size
mdadm --grow --raid-devices=3 /dev/md127 --backup-file /store/4TB_WD/md127.backup # reshape array removing 1 disk.

This has now completed:

cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md127 : active raid5 sdd1 sdc1(S) sdb1
      35156183040 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/2] [_UU]
      bitmap: 103/131 pages [412KB], 65536KB chunk

unused devices:

but has left me with a 3 disk degraded RAID5 with 2 active disks and one spare:

mdadm -D /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Fri Sep  9 22:39:53 2022
        Raid Level : raid5
        Array Size : 35156183040 (32.74 TiB 36.00 TB)
     Used Dev Size : 17578091520 (16.37 TiB 18.00 TB)
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Fri Jan 20 11:12:10 2023
             State : active, degraded
    Active Devices : 2
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 1

            Layout : left-symmetric
        Chunk Size : 64K

Consistency Policy : bitmap

              Name : oldserver-h.oldserver.lan:127
              UUID : 589dd683:d9945b24:768d9b2b:28441f90
            Events : 555962

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       1       8       49        1      active sync   /dev/sdd1
       2       8       17        2      active sync   /dev/sdb1

       3       8       33        -      spare   /dev/sdc1

How do I make this spare disk active so the array can rebuild to a healthy state? cat /sys/block/md127/md/sync_action shows idle and echoing repair into it does nothing. As a follow up, where did I go wrong in the first place? [edit] Adding output to lsblk as requested:

lsblk
NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                8:0    0   100G  0 disk
├─sda1             8:1    0     1G  0 part  /boot
└─sda2             8:2    0    99G  0 part
  ├─clearos-root 253:0    0  91.1G  0 lvm   /
  └─clearos-swap 253:1    0   7.9G  0 lvm   [SWAP]
sdb                8:16   0  16.4T  0 disk
└─sdb1             8:17   0  16.4T  0 part
  └─md127          9:127  0  32.8T  0 raid5 /store/RAID_A
sdc                8:32   0  16.4T  0 disk
└─sdc1             8:33   0  16.4T  0 part
  └─md127          9:127  0  32.8T  0 raid5 /store/RAID_A
sdd                8:48   0  16.4T  0 disk
└─sdd1             8:49   0  16.4T  0 part
  └─md127          9:127  0  32.8T  0 raid5 /store/RAID_A
sde                8:64   0   3.7T  0 disk
└─sde1             8:65   0   3.7T  0 part  /store/4TB_WD
sdf                8:80   0 931.5G  0 disk
└─sdf1             8:81   0 931.5G  0 part  /store/1TB1
sdg                8:96   0 931.5G  0 disk
└─sdg1             8:97   0 931.5G  0 part  /store/1TB2
sr0               11:0    1   1.2G  0 rom

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