Ok, so I did my weekly ritual of dismounting one of my western digital passports and replacing it with another (then taking the dismounted one to my offsite storage).
The new drive (pool name “rock”) mounted to /mnt/local/user/Z rather than /mnt/rock, This had me really puzzled because I could work with it in the GUI but on the command line /mnt/rock was not there (but the prior disk, “scissors” showed up in /mnt/scissors even though it was no longer mounted). Yet my replication task ran just fine.
I had to do a df on the command line to figure out what was going on. If there’s any way to set or even see the mount path in the GUI, I can’t find it. [there’s no button on the storage dashboard to look at any sort of settings other than for scrub tasks].
exporting rock and re-importing scissors had the same result, pool mounted to /mnt/local/user/Z. (What would happen if I mounted TWO pools if they both want to go to Z?)
I have scripts that run that assume the /mnt/rock path, so I’d like to get it changed back to that.
The more important question is why stuff like this always happens right before I go on a trip. 
Edit to add: I was using the command line to check for read-only being set, see Something is setting my data sets to read only - TrueNAS General - TrueNAS Community Forums for why I was doing that…making sure it wasn’t bollixed up like the other data sets were.
Another edit: I should think there would be some way to determine what the mount path is through the GUI. But the pool properties show only the name of the pool as its location.