find_and_modify isn't updating an object in the identity map. It's fairly easy to see, just do something like this:

band = Band.create(name: "Tool")
Band.where(_id: band.id).find_and_modify(:$push => {notes: {text: 'Hello'}})
band.reload()
band.notes.length
# => 1
Band.find(band.id).notes.length
# => 0