Do not set the env var globally, but use ENV.fetch

config/environments/test.rb

@@ -6,11 +6,6 @@ require 'gitlab/testing/request_inspector_middleware'
 require 'gitlab/testing/clear_process_memory_cache_middleware'
 require 'gitlab/utils'
 
-# Set GITLAB_USE_MODEL_LOAD_BALANCING to a non-nil value
-# CI tables consistently use the same connection
-# See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/82562
-ENV['GITLAB_USE_MODEL_LOAD_BALANCING'] ||= 'true'
-
 Rails.application.configure do
   # Make sure the middleware is inserted first in middleware chain
   config.middleware.insert_before(ActionDispatch::Static, Gitlab::Testing::RequestBlockerMiddleware)

lib/gitlab/database/load_balancing/setup.rb

@@ -90,7 +90,8 @@ module Gitlab
 
           def use_model_load_balancing?
             # Cache environment variable and return env variable first if defined
-            use_model_load_balancing_env = Gitlab::Utils.to_boolean(ENV["GITLAB_USE_MODEL_LOAD_BALANCING"])
+            default_use_model_load_balancing_env = Gitlab.dev_or_test_env? || nil
+            use_model_load_balancing_env = Gitlab::Utils.to_boolean(ENV.fetch('GITLAB_USE_MODEL_LOAD_BALANCING', default_use_model_load_balancing_env))
 
             unless use_model_load_balancing_env.nil?
               return use_model_load_balancing_env

spec/lib/gitlab/database/load_balancing/setup_spec.rb

@@ -274,6 +274,8 @@ RSpec.describe Gitlab::Database::LoadBalancing::Setup do
       end
 
       before do
+        allow(Gitlab).to receive(:dev_or_test_env?).and_return(false)
+
         # Rewrite `class_attribute` to use rspec mocking and prevent modifying the objects
         allow_next_instance_of(described_class) do |setup|
           allow(setup).to receive(:configure_connection)