Avoid touching the MR status if MR is not opened

app/models/merge_request.rb

@@ -231,10 +231,6 @@ class MergeRequest < ApplicationRecord
     end
   end
 
-  def merge_ref_auto_sync_enabled?
-    Feature.enabled?(:merge_ref_auto_sync, project, default_enabled: true)
-  end
-
   # Use this method whenever you need to make sure the head_pipeline is synced with the
   # branch head commit, for example checking if a merge request can be merged.
   # For more information check: https://gitlab.com/gitlab-org/gitlab-ce/issues/40004

app/services/merge_requests/mergeability_check_service.rb

@@ -4,7 +4,7 @@ module MergeRequests
   class MergeabilityCheckService < ::BaseService
     include Gitlab::Utils::StrongMemoize
 
-    delegate :project, :merge_ref_auto_sync_enabled?, to: :@merge_request
+    delegate :project, to: :@merge_request
     delegate :repository, to: :project
 
     def initialize(merge_request)
@@ -39,6 +39,10 @@ module MergeRequests
         return ServiceResponse.error(message: 'Merge ref is outdated due to disabled feature')
       end
 
+      unless payload.fetch(:merge_ref_head)
+        return ServiceResponse.error(message: 'Merge ref cannot be updated')
+      end
+
       ServiceResponse.success(payload: payload)
     end
 
@@ -89,6 +93,7 @@ module MergeRequests
     # Returns true if the merge-ref does not exists or is out of sync.
     def outdated_merge_ref?
       return false unless merge_ref_auto_sync_enabled?
+      return false unless merge_request.open?
 
       return true unless ref_head = merge_request.merge_ref_head
       return true unless target_sha = merge_request.target_branch_sha
@@ -107,5 +112,9 @@ module MergeRequests
       result = MergeRequests::MergeToRefService.new(project, merge_request.author).execute(merge_request)
       result[:status] == :success
     end
+
+    def merge_ref_auto_sync_enabled?
+      Feature.enabled?(:merge_ref_auto_sync, project, default_enabled: true)
+    end
   end
 end

spec/services/merge_requests/mergeability_check_service_spec.rb

@@ -228,6 +228,30 @@ describe MergeRequests::MergeabilityCheckService do
         end
       end
 
+      context 'when MR is marked as mergeable, but repo is not mergeable and MR is not opened' do
+        before do
+          # Making sure that we don't touch the merge-status after
+          # the MR is not opened any longer. Source branch might
+          # have been removed, etc.
+          allow(merge_request).to receive(:broken?) { true }
+          merge_request.mark_as_mergeable!
+          merge_request.close!
+        end
+
+        it 'returns ServiceResponse.error' do
+          result = subject
+
+          expect(result).to be_a(ServiceResponse)
+          expect(result.error?).to be(true)
+          expect(result.message).to eq('Merge ref cannot be updated')
+          expect(result.payload).to be_empty
+        end
+
+        it 'does not change the merge status' do
+          expect { subject }.not_to change(merge_request, :merge_status).from('can_be_merged')
+        end
+      end
+
       context 'when MR is mergeable but merge-ref does not exists' do
         before do
           merge_request.mark_as_mergeable!