bundler is complaining about stringio, but I don't even have that in my Gemfile.lock

Question

I’m building a Rails 6.1.4.4 app and trying to start my unicorn server (using a script that calls unicorn on /usr/local/rvm/wrappers/cfs-web/unicorn) and I keep getting this error

/usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/runtime.rb:309:in `check_for_activated_spec!': You have already activated stringio 3.0.0, but your Gemfile requires stringio 3.0.2. Since stringio is a default gem, you can either remove your dependency on it or try updating to a newer version of bundler that supports stringio as a default gem. (Gem::LoadError)
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/runtime.rb:25:in `block in setup'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/spec_set.rb:136:in `each'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/spec_set.rb:136:in `each'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/runtime.rb:24:in `map'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/runtime.rb:24:in `setup'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler.rb:151:in `setup'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/setup.rb:20:in `block in <top (required)>'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/ui/shell.rb:136:in `with_level'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/ui/shell.rb:88:in `silence'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/bundler-2.3.8/lib/bundler/setup.rb:20:in `<top (required)>'
    from <internal:/usr/local/rvm/rubies/ruby-3.0.2/lib/ruby/site_ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
    from <internal:/usr/local/rvm/rubies/ruby-3.0.2/lib/ruby/site_ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
    from /var/www/my-project/releases/20220512204117/config/boot.rb:3:in `<top (required)>'
    from /var/www/my-project/releases/20220512204117/config/application.rb:1:in `require_relative'
    from /var/www/my-project/releases/20220512204117/config/application.rb:1:in `<top (required)>'
    from /var/www/my-project/releases/20220512204117/config/environment.rb:2:in `require_relative'
    from /var/www/my-project/releases/20220512204117/config/environment.rb:2:in `<top (required)>'
    from <internal:/usr/local/rvm/rubies/ruby-3.0.2/lib/ruby/site_ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
    from <internal:/usr/local/rvm/rubies/ruby-3.0.2/lib/ruby/site_ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
    from config.ru:4:in `block in <main>'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/rack-2.2.3/lib/rack/builder.rb:125:in `instance_eval'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/rack-2.2.3/lib/rack/builder.rb:125:in `initialize'
    from config.ru:1:in `new'
    from config.ru:1:in `<main>'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/unicorn-6.1.0/lib/unicorn.rb:54:in `eval'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/unicorn-6.1.0/lib/unicorn.rb:54:in `block in builder'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/unicorn-6.1.0/lib/unicorn/http_server.rb:821:in `build_app!'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/unicorn-6.1.0/lib/unicorn/http_server.rb:140:in `start'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/gems/unicorn-6.1.0/bin/unicorn:128:in `<top (required)>'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/bin/unicorn:23:in `load'
    from /usr/local/rvm/gems/ruby-3.0.2@my-project/bin/unicorn:23:in `<main>'

I don’t have “stringio” referenced anywhere in my Gemfile.lock. Not sure where else to start looking. I have

$ echo $GEM_HOME
/usr/local/rvm/gems/ruby-3.0.2
$ echo $GEM_PATH
/usr/local/rvm/gems/ruby-3.0.2:/usr/local/rvm/gems/ruby-3.0.2@global

Not sure what to do to attempt to resolve this.