only inject preamble when the plugin is actually getting installed

lib/bundler/multilock.rb

@@ -924,6 +924,16 @@ def cli
         CLI.instance
       end
 
+      # @!visibility private
+      # Whether plugins.rb is being loaded by `bundle plugin install` (or a
+      # Gemfile's `plugin` command), as opposed to being loaded from the plugin
+      # index for some other bundle command
+      def installing_plugin?
+        caller_locations.any? do |location|
+          location.base_label == "register_plugin" && location.path.end_with?("bundler/plugin.rb")
+        end
+      end
+
       # @!visibility private
       def cli_loaded?
         Bundler.autoload?(:CLI).nil? && !defined?(Bundler::CLI).nil?
@@ -1226,7 +1236,9 @@ def write_lockfile(lockfile_definition,
   end
 end
 
-Bundler::Multilock::Preamble.inject unless Bundler::Multilock.loaded? || Bundler.frozen_bundle?
+if Bundler::Multilock.installing_plugin? && !Bundler::Multilock.loaded? && !Bundler.frozen_bundle?
+  Bundler::Multilock::Preamble.inject
+end
 
 if Bundler::Multilock.cli_loaded?
   require_relative "multilock/ext/cli"

spec/bundler/multilock/preamble_spec.rb

@@ -423,4 +423,30 @@
       expect(File.read("Gemfile")).to eq gemfile
     end
   end
+
+  it "does not inject the preamble into an alternate Gemfile that's loaded from the plugin index" do
+    with_gemfile(<<~RUBY) do
+      lockfile("alt", gemfile: "alt/Gemfile", lockfile: "alt/Gemfile.lock")
+    RUBY
+      FileUtils.mkdir_p("alt")
+      alt_gemfile = <<~RUBY
+        source "https://rubygems.org"
+      RUBY
+      File.write("alt/Gemfile", alt_gemfile)
+
+      # the preamble is expected in the main Gemfile after installing the plugin
+      local_path = Shellwords.escape(plugin_path)
+      invoke_bundler("plugin install bundler-multilock --path=#{local_path}")
+      expect(File.read("Gemfile")).to include("bundler-multilock")
+
+      # the plugin index lives in the app config directory, which is normally next to the Gemfile.
+      # Pointing at the main one makes the plugin load for every bundle command, even when
+      # the Gemfile doesn't mention it
+      invoke_bundler("list",
+                     env: { "BUNDLE_GEMFILE" => "alt/Gemfile",
+                            "BUNDLE_APP_CONFIG" => File.join(Dir.pwd, ".bundle") })
+
+      expect(File.read("alt/Gemfile")).to eq alt_gemfile
+    end
+  end
 end