Seems like a mighty big gotchya with RaidZ expansion.
This is the relevant bug report
https://ixsystems.atlassian.net/browse/NAS-131728
Basically, the space available is calculated based on the original raidz width and 128k block size.
This means that it is always going to be an underestimate once you add additionally disks.
Oops.
And that’s bascally all there is to it.
I’m not sure of the consequences, but this seems problematic… and perhaps should be taken care of in the UI at least… if it can’t be fixed in ZFS proper
So, 33GB / (3/5s) = 55GB.
That is not something the user should have to do in their head.