Describing in detail the history of the pool. I can infer you have replaced drives and used raidz expansion while the pool was already filled. Both of these processes have issues in SCALE.
But 5*4 TB would give 20 TB, so it seems that autoexpand did succeed. (sudo lsblk can confirm)
So this must be purely the vdev expansion thing…
Good news: Capacity is there, just not properly reported.
Bad news: The fundamental issue lies upstream in OpenZFS, and is unlikely to be addressed any time soon. The only possible fix is… “backup-destroy-restore”.