ZFS ashift on vdev addition for pre-12 pools

Regarding this thread: https://www.truenas.com/community/threads/cannot-replace-disks-in-pool.91490/

I ran into a an issue where the GUI could not replace a faulty disk with the error "already in replacing/spare config; wait for completion or use 'zpool detach'". Sorry, i don't have the exact additional error-Message from the GUI anymore.

There was no other replacing in progress. And the pool does not have a spare-disk anymore (there was one until a year or so back).

The pool itself is dating back to FreeNAS 9 which was expanded with one additional VDEV (raidz2-4) under TrueNAS 12.0. Currently running TrueNAS 12.0-U2.

1. zpool status -v ggmtank01
   pool: ggmtank01
   state: DEGRADED
   status: One or more devices has been taken offline by the administrator.
   Sufficient replicas exist for the pool to continue functioning in a
   degraded state.
   action: Online the device using 'zpool online' or replace the device with
   'zpool replace' .
   scan: scrub repaired 8K in 07 :32:07 with 0 errors on Sun Aug 29 07 :32:37 2021
   config:

NAME STATE READ WRITE CKSUM
ggmtank01 DEGRADED 0 0 0
raidz2-0 ONLINE 0 0 0
gptid/071d138c-9644-11e8-8380-000743400660 ONLINE 0 0 0
gptid/07d35682-9644-11e8-8380-000743400660 ONLINE 0 0 0
gptid/ef627048-743e-11eb-8d93-e4434bb19fe0 ONLINE 0 0 0
gptid/167a10f2-7aa8-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/e82432e5-8585-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
raidz2-1 ONLINE 0 0 0
gptid/73340a80-5449-11e9-b326-000743400660 ONLINE 0 0 0
gptid/49297ee3-00c5-11ec-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/625036be-8586-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/44286cae-7aa9-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/3ffd3cdc-7440-11eb-8d93-e4434bb19fe0 ONLINE 0 0 0
raidz2-2 ONLINE 0 0 0
gptid/c071d681-743c-11eb-8d93-e4434bb19fe0 ONLINE 0 0 0
gptid/0f702ce5-9644-11e8-8380-000743400660 ONLINE 0 0 0
gptid/d1bdee26-78df-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/f3bcbd88-7aa9-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/0826e283-8587-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
raidz2-4 DEGRADED 0 0 0
gptid/8aaceda5-9ea7-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/8b2fc90b-9ea7-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/8c2ee1c1-9ea7-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/8bf70e18-9ea7-11eb-bdc3-e4434bb19fe0 ONLINE 0 0 0
gptid/8c8ddaa8-9ea7-11eb-bdc3-e4434bb19fe0 OFFLINE 0 0 0
logs
mirror-3 ONLINE 0 0 0
gptid/123e1981-9644-11e8-8380-000743400660 ONLINE 0 0 0
gptid/12b0bdb1-9644-11e8-8380-000743400660 ONLINE 0 0 0
cache
gptid/f4918c31-ff0f-11e9-b449-000743400660 ONLINE 0 0 0

errors: No known data errors

zpool reported the following ashift for the Pool:

1. zpool get ashift ggmtank01
   NAME PROPERTY VALUE SOURCE
   ggmtank01 ashift 0 default

But zdb shows the following ashift-values for the Pool, and it is different for the newly added VDEV (children[4])! I am pretty sure i haven't changed any Default-settings when adding the new VDEV - but that's a couple of months back!

1. zdb -C -U /data/zfs/zpool.cache
   (truncated)
   ggmtank01:
   vdev_tree:
   children[0]:
   type: 'raidz'
   nparity: 2
   ashift: 12
   children[1]:
   type: 'raidz'
   nparity: 2
   ashift: 12
   children[2]:
   type: 'raidz'
   nparity: 2
   ashift: 12
   children[3]:
   type: 'mirror'
   ashift: 12
   children[4]:
   type: 'raidz'
   nparity: 2
   ashift: 9

Nonetheless - thanks to the mentioned thread - i was able to replace the faulted disk with the following statement and the pool is currently resilvering.

zpool replace -o ashift=9 ggmtank01 gptid/<faulty-rawuuid> gptid/<new-rawuuid>

I am not sure, if this is a bug or simply a bad GUI message?