Bundler unwanted downgrades

bundle_env.md

Environment

Bundler       1.16.1
  Platforms   ruby, x86_64-darwin-17
Ruby          2.4.3p205 (2017-12-14 revision 61247) [x86_64-darwin17]
  Full Path   /Users/flo/.rbenv/versions/2.4.3/bin/ruby
  Config Dir  /Users/flo/.rbenv/versions/2.4.3/etc
RubyGems      2.6.14
  Gem Home    /Users/flo/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0
  Gem Path    /Users/flo/.gem/ruby/2.4.0:/Users/flo/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0
  User Path   /Users/flo/.gem/ruby/2.4.0
  Bin Dir     /Users/flo/.rbenv/versions/2.4.3/bin
Tools         
  Git         2.13.1
  RVM         not installed
  rbenv       rbenv 1.1.0

  chruby      not installed

repro-6529.sh

#!/bin/bash

set -ex

mkdir -p /tmp/repro-6529
cd /tmp/repro-6529

bundle --version

cat > Gemfile <<EOF
source 'https://rubygems.org'

gem 'mongo'
gem 'mongoid'
gem 'carrierwave'
gem 'carrierwave-mongoid'
EOF

cat > Gemfile.lock <<EOF
GEM
  remote: https://rubygems.org/
  specs:
    activemodel (5.2.0)
      activesupport (= 5.2.0)
    activesupport (5.2.0)
      concurrent-ruby (~> 1.0, >= 1.0.2)
      i18n (>= 0.7, < 2)
      minitest (~> 5.1)
      tzinfo (~> 1.1)
    bson (4.3.0)
    carrierwave (1.2.2)
      activemodel (>= 4.0.0)
      activesupport (>= 4.0.0)
      mime-types (>= 1.16)
    carrierwave-mongoid (1.0.0)
      carrierwave (>= 0.8, < 1.3)
      mongoid (>= 3.0, < 7.0)
      mongoid-grid_fs (>= 1.3, < 3.0)
    concurrent-ruby (1.0.5)
    i18n (1.0.1)
      concurrent-ruby (~> 1.0)
    mime-types (3.1)
      mime-types-data (~> 3.2015)
    mime-types-data (3.2016.0521)
    minitest (5.11.3)
    mongo (2.5.3)
      bson (>= 4.3.0, < 5.0.0)
    mongoid (6.4.1)
      activemodel (>= 5.1, < 6.0.0)
      mongo (>= 2.5.1, < 3.0.0)
    mongoid-grid_fs (2.3.0)
      mime-types (>= 1.0, < 4.0)
      mongoid (>= 3.0, < 7.0)
    thread_safe (0.3.6)
    tzinfo (1.2.5)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  carrierwave
  carrierwave-mongoid
  mongo
  mongoid

BUNDLED WITH
   1.16.1
EOF

bundle install

# Expected behaviour: should not downgrade carrierwave-mongoid
# Observed behaviour: downgrades carrierwave-mongoid to an really old version

BUNDLE_ONLY_UPDATE_TO_NEWER_VERSIONS=true bundle update mongoid carrierwave-mongoid