damn @arj beat me to it (and I didn't see because of replication delays!)

following this down I was I got to looking at that minimal.js too

  var codec = {
    encode: function (obj) {
      return JSON.stringify(obj, null, 2)
    },
    decode: function (str) {
      return unbox(JSON.parse(str.toString()), unboxers)
    },
    buffer: false,    ////  << watch this value
    type: 'ssb'
  }

  var log = OffsetLog(path.join(dirname, 'log.offset'), {blockSize:1024*16, codec:codec})

diving into flumelog-offset/inject.js (where this codec object is used) :

      var data = {
        value: codec.decode(codec.buffer ? value : value.toString()),
        prev: prev,
        next: next
      }

I didn't dig further to check that value is a Buffer...
but seems like arj is correct value -> toString -> toString() -> JSON.parse -> unbox (toString twice!)