Bundler compact index fails to find gem version that exists

Environment

Bundler version: 4.0.6 (also tested on 2.5.23, which has the same behavior)
RubyGems version: 4.0.6
Ruby version: 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [arm64-darwin25]
Platform: arm64-darwin25 (macOS)

Summary

Bundler cannot find minitest-bonus-assertions version 3.0 when using the compact index, even though the version exists on RubyGems.org and is listed in the compact index. Using --full-index resolves the issue.

Steps to Reproduce

1. Create a Gemfile with:

  source "https://rubygems.org"
  gem "minitest-bonus-assertions", ">= 3.0"

2. Run bundle update --all

  Fetching gem metadata from https://rubygems.org/.........
  Could not find gem 'minitest-bonus-assertions (>= 3.0)' in rubygems repository https://rubygems.org/, cached gems or
  installed locally or in gems cached in vendor/cache.

  The source contains the following gems matching 'minitest-bonus-assertions':
    * minitest-bonus-assertions-1.0
    * minitest-bonus-assertions-2.0

Diagnostic Information

1. The gem exists and is not yanked:
   • https://rubygems.org/gems/minitest-bonus-assertions shows version 3.0 (released April 26, 2017)
2. gem fetch works correctly:

  gem fetch minitest-bonus-assertions

3. gem specification shows version 3.0:

  gem specification minitest-bonus-assertions -r | grep "version:"
  version: !ruby/object:Gem::Version
    version: '3.0'

4. The compact index contains all three versions:

  curl -s "https://index.rubygems.org/versions" | grep "minitest-bonus-assertions"
  minitest-bonus-assertions 1.0,2.0,3.0 2685d709a56e0930775f271d23305ed2

5. Gem metadata:

    - required_ruby_version: >= 2.0
    - required_rubygems_version: >= 0
    - specification_version: 4
    - rubygems_version: 2.5.1 (version used to build the gem)

Workaround

Using --full-index resolves the issue:

  bundle update --all --full-index

Reproduce the failure with the provided Gemfile using bundle update --all, then compare it with --full-index. Start by tracing Bundler's compact-index resolution for minitest-bonus-assertions and the versions endpoint; done means version 3.0 resolves through the compact index without requiring the workaround.