There is an old expression:
Never attribute to malice that which is adequately explained by stupidity.
Hanlon's razor - Wikipedia
Ideally, their would be a check list of items to verify after major GUI changes. Perhaps multiple hot spares was simply overlooked as something to check. Of course now is the time to add that to the check list, (and or automated GUI testing).
Since no one else did it, I created a bug on the issue:
Hopefully mine will be closed as a duplicate, (meaning iX / TrueNAS people already know about this problem). But, if not, they do now.
PS: You may have to log into Jira to see the bug