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I have the same problem with my lamium. I've seen earwigs crawling on the soil, so I'm blaming those ugly, scum-of-the-earth insects. Granular ant killer (Terro) has helped somewhat, but I hate to use too much of it.
I don't know what causes it but I wanted to let you know that the same thing happens to my lamium every year. The plants around it are not affected so it's something to which the lamium is particularly susceptible.
I've noticed the slugs/snails do seem to prefer some plants vs others--this year for example the snails have shredded the leaves of my Acanthus, but haven't so much as touched anything around it. So it's possible they find Lamium yummier than other things.