This bug was originally reported (#944743) against 6.0.0-1 and should have been fixed in 6.0.0-2 (by passing dh_installsystemduser --no-enable).
Installing 6.0.3 in a VM today appears to install but not auto-enable the user unit. Can you confirm if this was an upgrade or a clean install? Unfortunately, there doesn't appear to be any way to distinguish retrospectively between automatically-enabled and retrospectively-enabled units, and I'm somewhat loathe to ship a maintscript which disables it automatically as a result (like https://bugs.launchpad.net/ubuntu/+source/rygel/+bug/1848692 ). You can disable it for all users with systemctl --user --global disable jupyter-notebook