ZFS - reboot during resilver doesn't work

If you have a ZFS mirror you're supposed to be able to reboot while it's 
resilvering. It's a background job and should just continue where it 
left off, more or less. I'm pretty sure I've done this in the past, with 
the system on the pool being resilvered too.

But I've noticed it doesn't quite work (14.3-RELEASE) as it won't shut 
down. The disk activity LED is jammed on. It syncs the buffers, but 
carries accessing the disks with an unresponsive terminal. The only 
unusual thing is that at least one of the drives is SMR (don't ask!)

If you do shut it down hard (kill the power) it restarts the 
resilvering, but having taken several steps backwards.

Has anyone else noticed this?

Why do I care? I don't like to leave a distant data centre without 
restarting something a few times to make sure I can reboot safely later on.

Thanks, Frank.