My bet would be a slug...The reason that I think so was that I made the mistake (never again) of putting my Dendrobium apallon outside last year in the summer, and it was like nectar for the slugs and snails; they decapitated it...Surprisingly (and thankfully) it has recovered now. I even have to be careful with open windows because the Dendrobium must emit some kind of scent that draws the pests near!! ;P I despise slugs and snails, and I often put bait down. Also I have heard that copper tape is good, and I am considering getting some to apply to my pots. I am sorry about the damage to your Max. tenuifolia though. Perhaps repot it to make sure that no mollusks or other pests are hiding in the bark? (They sometimes do).
(As your plant has not been outside, the damage could have been inflicted by a mollusk or other pest that has hatched out from the inside. )