Just run that to see the bug being replicated.

collection_image_bug.rb

require 'shopify_api'

shop = ShopifyAPI::Session.new('ultraexportimport.myshopify.com', '****************************')
ShopifyAPI::Base.activate_session(shop)

begin

  ########################################################
  # 1 - Create new Collection
  ########################################################

  data = {
      "custom_collection": {
          "handle": "custom-collection-#{rand(1000)}",
          "title": "Custom Collection",
          "image": {"src": "http://cdn3-www.cattime.com/assets/uploads/2011/08/best-kitten-names-1.jpg"
          }
      }
  }

  entity = ShopifyAPI::CustomCollection.new(data)

  result = entity.save

  if result
    puts "Created:"
    puts JSON.pretty_generate(JSON.parse(entity.to_json))
  else
    if entity.errors
      puts
      puts "Errors:"
      puts JSON.pretty_generate(JSON.parse(entity.errors.to_json))
    else
      puts
      puts "Don't know what happened - didnt't save, but there are also no errors..."
    end
    abort
  end

  puts
  puts "========================================"
  puts "Check that image link from loaded Collection - it works!"
  puts "After you will load and simply save the same entity, it will not work anymore, and image will be lost, and entity saving will fail."
  puts "Press Enter to see what happens after save..."
  gets

  id = entity.id


  ########################################################
  # 2 - Load that same Collection by ID from Shopify
  ########################################################

  puts
  puts "========================================"
  puts "Searching by ID: #{id}"

  entity = ShopifyAPI::CustomCollection.find(id)

  puts
  puts "========================================"
  puts "Loaded:"
  puts JSON.pretty_generate(JSON.parse(entity.to_json))

  sleep 1 # Especially, when sleeping here, the issue repeats more often

  puts "========================================"
  puts "Saving without any changes..."

  result = entity.save

  if result
    puts "Saved:"
    puts JSON.pretty_generate(JSON.parse(entity.to_json))
    puts
    puts "OK, it saved this time - but please, repeat the same script several times, and quite often it will fail saving."
  else
    if entity.errors
      puts
      puts "Errors:"
      puts JSON.pretty_generate(JSON.parse(entity.errors.to_json))
    else
      puts
      puts "Don't know what happened - didnt't save, but there are also no errors..."
    end
    abort
  end

rescue => e
  puts
  puts "Error:"
  puts e
end