A new logical volume appearing after reducing the size of another

I was following these instructions to remove one disk from a 3-disk logical volume, and at some point a new logical volume appeared:

ls /dev/mapper control ubuntu--vg-root ubuntu--vg-swap_1 vgdata-lvdata vgdata-pvmove0

The last one appeared after reducing the size of vgdata-lvdata. Why is that? I cannot see it anywhere when running lvdisplay. Is it related to the pvmove command I ran? Is this a problem?

Is it related to the pvmove command I ran?

Yes. pvmove LVs are temporary, and managed by pvmove. LVM tools try to hide them. That device mapper reference is harder to hide, other things use dm in addition to LVM.

Moves can take a long time to complete. Check for any running pvmove process doing I/O. Run pvmove with no arguments to resume any stopped move. If pvmove finishes without error, consider the process successful. Any stray pvmove mapper devices are not a concern. They should go away at the next regularly scheduled reboot, at the latest.

After the move completes, vgreduce should exclude this PV from the VG without complaint.