Doesn't look like sunburn to me, which would manifest much more evenly over entire leaves. Damage originating far down the sides, or in the middle of leaves indicates possible grazing by some kind of plant muncher.

Sans leaves take a long time for damage to manifest, so often the cause is mysterious. Whatever happened, it was a while ago. I would remove the damaged parts so I could tell if new damage is occurring. Could it have been mechanical damage when the plants were put there? When did you start to notice damage? How long have these plants been in this spot?

Do you have a closer pic of some damaged leaves you could add?