Ok, my current theory is this, please poke holes: autofs and nfs-common are both unpacked. Then the setup begins.
If autofs is setup first, it means systemctl daemon-reload will be called at the end, and that will run all the generators. Since nfs-common is already unpacked, it's generator is on disk, and will be run. But /etc/nfs.conf doesn't exist yet: it's produced by ucf in nfs-common's postinst. That means the generator will not be able to fetch the pipefs-directory config, and will just exit silently. But autofs is being started, and it requires rpc_pipefs.target, but at this time, that target unit is the one from the nfs-common package that will trigger the var-lib-nfs-rpc_pipefs.mount unit. Then nfs-common is setup. This time, it will produce /etc/nfs.conf, and when systemctl daemon-reload is called and the generator run, it will find /etc/nfs.conf, and the pipefs-directory setting, and see it's different from the default, and produce the generated target and mount files, which will mount rpc_pipefs in /run/rpc_pipefs now.
