Systemd debug log?

How to view the debug log message from systemd?
In systemd it has lines like this:

log_unit_debug(u, "Time change, recalculating next elapse.");

I tried setting SYSTEMD_LOG_LEVEL=debug but I only see something like dbus messages when I run systemd-run commands. (I also tried journalctl, but no entry for either transient unit or systemd.)

In case this is an XY problem: I’m trying to find #29245.

// Set user session systemd to debug
systemctl log-level debug --user
// 1924 is current user systemd's PID
journalctl -b _PID=1924 | less +G
// Or, to see user unit (--user-unit instead of -u)
journalctl -b --user-unit XXX | less +G
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