So, this is indeed a bug fix, but somewhat more involved. It only concerns the case where you run `juju deploy mysql --networks=foo,bar --constraints networks=good,^bad --to lxc:0`, i.e. creating a new (but "dirty") container for the unit. Since it's dirty the hard-coded unit assignment logic (AssignCleanEmpty) does not kick in to set the container's constraints. It does not happen with `--to 0` or without `--to`.

So I think the fix should stay?