I wonder if this is a case of something about the process/charm itself configuring ulimits. On LXD I definitely see 64 in both cases.
Even with ssh, I get:
ubuntu@juju-1f04ed-0:~$ ulimit -l
64
ubuntu@juju-1f04ed-0:~$ sudo -i
ulimit -l
root@juju-1f04ed-0:~# ulimit -l
64
As for LimitMEMLOCK being set, I know the systemd wrapper we have has support for changing memlock (and we set memlock to UNLIMITED for juju-db).
I don’t see other locations where juju is trying to control those things for unit agents, etc.
I also wonder if those are bionic vs focal vs xenial differences.