June Hughes writes
The peculiar thing is, it has only happened to a couple of stems. The
rest are fine.

M experience is that slugs return to the same leaf night after night, so
that they will completely denude one plant before moving on to the next.
The exception is something like a runner bean, where they find the stem
more amenable to attack than the bristly leaves - once they've chewed
through the stem and felled the plant, they move on to the next plant
rather than continuing to eat the fallen stem.

--
Kay