Rails before_update not triggering on update but save works

Question

Model looks like

class Campaign < ActiveRecord::Base
  before_update :delete_triggers

  def delete_triggers
    Trigger.where(campaign_id: id).delete_all
  end
end

and in the controller I do

Campaign.find(params[:campaignId]).update(campaign_params)

where campaign_params just grabs the body and does some cleanup. delete_triggers never gets called though. There is no filter (there will be, but I removed it since the method wasn't firing. It didn't make a difference). In order to sanity check, I added to the model

before_save :check_dirty

def check_dirty
  pp changed?
end

which prints true as expected.

What would cause before_update to not trigger on a call to .update, where before_save does? Obviously, I could just use before_save and add some filtering logic in the callback, but that's hacky when there's a hook made for this purpose.

(There are other questions similar to this, but not the same. One has a syntax error in a proc, and I'm not using a proc; and the other is a bad filter, which I removed specifically to avoid that possibility and am still getting the issue)