systemd per-user unit search path doesn't include .config/systemd/user?

Question

Systemd docs state that per-user units can be added to multiple locations: see Table 2 on the man page:

https://www.freedesktop.org/software/systemd/man/systemd.unit.html

This includes, for example, ~/.config/systemd/user.

However, on my Ubuntu 18.04 box, systemd doesn't appear to look there, only at a subset of locations (... as it's apparent using strace on the systemd process while trying to start my service):

openat(AT_FDCWD, "/run/user/1000/systemd/system.control/parsec.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/run/user/1000/systemd/transient/parsec.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/systemd/user/parsec.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/lib/systemd/user/parsec.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)

Why? Also, how can I get it to pick up units from user-writable locations?