diff --git a/changes.d/6448.fix.md b/changes.d/6448.fix.md
new file mode 100644
index 0000000000..56a0a44f3c
--- /dev/null
+++ b/changes.d/6448.fix.md
@@ -0,0 +1 @@
+Fix the non-spawning of parentless sequential xtriggered tasks when outputs are set.
diff --git a/cylc/flow/task_pool.py b/cylc/flow/task_pool.py
index 6d320a7301..6a83adefbd 100644
--- a/cylc/flow/task_pool.py
+++ b/cylc/flow/task_pool.py
@@ -1948,6 +1948,9 @@ def set_prereqs_and_outputs(
             if prereqs:
                 self._set_prereqs_itask(itask, prereqs, flow_nums)
             else:
+                # Spawn as if seq xtrig of parentless task was satisfied,
+                # with associated task producing these outputs.
+                self.check_spawn_psx_task(itask)
                 self._set_outputs_itask(itask, outputs)
 
         # Spawn and set inactive tasks.
@@ -2071,21 +2074,7 @@ def remove_tasks(self, items):
         itasks, _, bad_items = self.filter_task_proxies(items)
         for itask in itasks:
             # Spawn next occurrence of xtrigger sequential task.
-            if (
-                itask.is_xtrigger_sequential
-                and (
-                    itask.identity not in
-                    self.xtrigger_mgr.sequential_has_spawned_next
-                )
-            ):
-                self.xtrigger_mgr.sequential_has_spawned_next.add(
-                    itask.identity
-                )
-                self.spawn_to_rh_limit(
-                    itask.tdef,
-                    itask.tdef.next_point(itask.point),
-                    itask.flow_nums
-                )
+            self.check_spawn_psx_task(itask)
             self.remove(itask, 'request')
         if self.compute_runahead():
             self.release_runahead_tasks()
@@ -2134,26 +2123,12 @@ def _force_trigger(self, itask):
                 itask.tdef.next_point(itask.point),
                 itask.flow_nums
             )
-        # Task may be set running before xtrigger is satisfied,
-        # if so check/spawn if xtrigger sequential.
-        elif (
-            itask.is_xtrigger_sequential
-            and (
-                itask.identity not in
-                self.xtrigger_mgr.sequential_has_spawned_next
-            )
-        ):
-            self.xtrigger_mgr.sequential_has_spawned_next.add(
-                itask.identity
-            )
-            self.spawn_to_rh_limit(
-                itask.tdef,
-                itask.tdef.next_point(itask.point),
-                itask.flow_nums
-            )
         else:
             # De-queue it to run now.
             self.task_queue_mgr.force_release_task(itask)
+        # Task may be set running before xtrigger is satisfied,
+        # if so check/spawn if xtrigger sequential.
+        self.check_spawn_psx_task(itask)
 
     def force_trigger_tasks(
         self, items: Iterable[str],
@@ -2237,10 +2212,23 @@ def spawn_parentless_sequential_xtriggers(self):
         """Spawn successor(s) of parentless wall clock satisfied tasks."""
         while self.xtrigger_mgr.sequential_spawn_next:
             taskid = self.xtrigger_mgr.sequential_spawn_next.pop()
-            self.xtrigger_mgr.sequential_has_spawned_next.add(taskid)
             itask = self._get_task_by_id(taskid)
-            # Will spawn out to RH limit or next parentless clock trigger
-            # or non-parentless.
+            self.check_spawn_psx_task(itask)
+
+    def check_spawn_psx_task(self, itask: 'TaskProxy') -> None:
+        """Check and spawn parentless sequential xtriggered task (psx)."""
+        # Will spawn out to RH limit or next parentless clock trigger
+        # or non-parentless.
+        if (
+            itask.is_xtrigger_sequential
+            and (
+                itask.identity not in
+                self.xtrigger_mgr.sequential_has_spawned_next
+            )
+        ):
+            self.xtrigger_mgr.sequential_has_spawned_next.add(
+                itask.identity
+            )
             self.spawn_to_rh_limit(
                 itask.tdef,
                 itask.tdef.next_point(itask.point),
diff --git a/tests/integration/test_sequential_xtriggers.py b/tests/integration/test_sequential_xtriggers.py
index 1c98437bc7..4ef563a1c5 100644
--- a/tests/integration/test_sequential_xtriggers.py
+++ b/tests/integration/test_sequential_xtriggers.py
@@ -104,6 +104,26 @@ async def test_trigger(sequential, start):
         assert list_cycles(sequential) == ['2000', '2001']
 
 
+async def test_set(sequential, start):
+    """It should spawn its next instance if outputs are set ahead of time.
+
+    If you set outputs of a sequentially spawned task before its xtriggers
+    have become satisfied, then the sequential spawning chain is broken.
+
+    The task pool should defend against this to ensure that setting outputs
+    doesn't cancel it's future instances and their downstream tasks.
+    """
+    async with start(sequential):
+        assert list_cycles(sequential) == ['2000']
+
+        foo = sequential.pool.get_task(ISO8601Point('2000'), 'foo')
+        # set foo:succeeded it should spawn next instance
+        sequential.pool.set_prereqs_and_outputs(
+            ["2000/foo"], ["succeeded"], None, ['all'])
+
+        assert list_cycles(sequential) == ['2001']
+
+
 async def test_reload(sequential, start):
     """It should set the is_xtrigger_sequential flag on reload.