try

cylc/flow/config.py

@@ -2258,7 +2258,6 @@ def load_graph(self):
                 task_output_opt=task_output_opt
             )
             parser.parse_graph(graph)
-            self.check_outputs(parser.task_output_opt.keys())
             task_output_opt.update(parser.task_output_opt)
             self.workflow_polling_tasks.update(
                 parser.workflow_state_polling_tasks)
@@ -2285,11 +2284,10 @@ def check_outputs(self, tasks_and_ouputs):
         safe
         """
         for task, output in tasks_and_ouputs:
-
             try:
                 registered_outputs = self.cfg['runtime'][task]['outputs']
             except KeyError:
-                pass
+                registered_outputs = []
             if (
                 not TaskOutputs.is_valid_std_name(output)
                 and output not in registered_outputs

tests/integration/test_optional_outputs.py

@@ -288,14 +288,11 @@ def implicit_completion_config(mod_flow, mod_validate):
         },
         'runtime': {
             'root': {
-                'outputs': {
-                    'x': 'xxx',
-                    'y': 'yyy',
-                    'z': 'zzz',
-                }
+                'outputs': {n: f'{n}{n}{n}' for n in 'abcdefghijklxyz'}
             }
         }
     })
+
     return mod_validate(id_)
 
 
@@ -322,13 +319,15 @@ async def test_implicit_completion_expression(
     implicit_completion_config,
     task,
     condition,
+    monkeypatch,
 ):
     """It should generate a completion expression from the graph.
 
     If no completion expression is provided in the runtime section, then it
     should auto generate one inferring whether outputs are required or not from
     the graph.
     """
+    # monkeypatch.setattr('cylc.flow.config.WorkflowConfig.check_outputs', lambda *_: None)
     completion_expression = get_completion_expression(
         implicit_completion_config.taskdefs[task]
     )