Update: I tried the approach @Christian Bundy recommended above in ssb-server@14.1.3:

  const unboxerMap = (msg, cb) => cb(null, msg)
  //const unboxerMap = (msg, cb) => cb(null, db.unbox(msg))

And regenerated indexes, but I was still able to view my private messages without my key present in the lite client.

Maybe I was doing something wrong, or this isn't quite the fix. I'm not quite sure yet.