When creating pool, adding vdev, then removing it, leaves debris

I have 4 unused disks I've just inserted in my FREENAS-CERTIFIED-2U-12

- go to Storage > Pools

- click Add

- chose 'create new pool'

- it detects the 4 available disks, yielding Image1.png

- checkmark a pair of equal disks

- click the right arrow to add them to the first vdev

- click the "Add Data" button to add a secend vdev

- checkmark the other pair of equal disks

- click the right arrow to add them to the second vdev

- so far so good, yielded Image2.png

- now, click the X button of the first vdev

(1)

It now shows, in red, a strangely worded error message:

"WARNING: Adding data vdevs with different numbers of disks is not recommended. First vdev has 0 disks, new vdev has 2."

There is no "first vdev", I just deleted it!

(2) The "Estimated total raw data capacity" still shows 4.54 TiB, which doesn't make sense after the removal of the first vdev.

See Image3.png